Allow dummy thermistors without pin definitions (#20159)

Marlin/src/inc/Conditionals_post.h

   #define HEATER_0_USES_THERMISTOR 1
   #if TEMP_SENSOR_0 == 1000
     #define HEATER_0_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_0 == 998 || TEMP_SENSOR_0 == 999
+    #define HEATER_0_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_0_MINTEMP
   #define HEATER_1_USES_THERMISTOR 1
   #if TEMP_SENSOR_1 == 1000
     #define HEATER_1_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_1 == 998 || TEMP_SENSOR_1 == 999
+    #define HEATER_1_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_1_MINTEMP
   #define HEATER_2_USES_THERMISTOR 1
   #if TEMP_SENSOR_2 == 1000
     #define HEATER_2_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_2 == 998 || TEMP_SENSOR_2 == 999
+    #define HEATER_2_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_2_MINTEMP
   #define HEATER_3_USES_THERMISTOR 1
   #if TEMP_SENSOR_3 == 1000
     #define HEATER_3_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_3 == 998 || TEMP_SENSOR_3 == 999
+    #define HEATER_3_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_3_MINTEMP
   #define HEATER_4_USES_THERMISTOR 1
   #if TEMP_SENSOR_4 == 1000
     #define HEATER_4_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_4 == 998 || TEMP_SENSOR_4 == 999
+    #define HEATER_4_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_4_MINTEMP
   #define HEATER_5_USES_THERMISTOR 1
   #if TEMP_SENSOR_5 == 1000
     #define HEATER_5_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_5 == 998 || TEMP_SENSOR_5 == 999
+    #define HEATER_5_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_5_MINTEMP
   #define HEATER_6_USES_THERMISTOR 1
   #if TEMP_SENSOR_6 == 1000
     #define HEATER_6_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_6 == 998 || TEMP_SENSOR_6 == 999
+    #define HEATER_6_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_6_MINTEMP
   #define HEATER_7_USES_THERMISTOR 1
   #if TEMP_SENSOR_7 == 1000
     #define HEATER_7_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_7 == 998 || TEMP_SENSOR_7 == 999
+    #define HEATER_7_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef HEATER_7_MINTEMP
   #define HEATER_BED_USES_THERMISTOR 1
   #if TEMP_SENSOR_BED == 1000
     #define HEATER_BED_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_BED == 998 || TEMP_SENSOR_BED == 999
+    #define HEATER_BED_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef BED_MINTEMP
   #define HEATER_CHAMBER_USES_THERMISTOR 1
   #if TEMP_SENSOR_CHAMBER == 1000
     #define HEATER_CHAMBER_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_CHAMBER == 998 || TEMP_SENSOR_CHAMBER == 999
+    #define HEATER_CHAMBER_DUMMY_THERMISTOR 1
   #endif
 #else
   #undef CHAMBER_MINTEMP
   #define HEATER_PROBE_USES_THERMISTOR 1
   #if TEMP_SENSOR_PROBE == 1000
     #define HEATER_PROBE_USER_THERMISTOR 1
+  #elif TEMP_SENSOR_PROBE == 998 || TEMP_SENSOR_PROBE == 999
+    #define HEATER_PROBE_DUMMY_THERMISTOR 1
   #endif
 #endif
 
 //
 // ADC Temp Sensors (Thermistor or Thermocouple with amplifier ADC interface)
 //
-#define HAS_ADC_TEST(P) (PIN_EXISTS(TEMP_##P) && TEMP_SENSOR_##P != 0 && DISABLED(HEATER_##P##_USES_MAX6675))
+#define HAS_ADC_TEST(P) (PIN_EXISTS(TEMP_##P) && TEMP_SENSOR_##P != 0 && NONE(HEATER_##P##_USES_MAX6675, HEATER_##P##_DUMMY_THERMISTOR))
 #if HAS_ADC_TEST(0)
   #define HAS_TEMP_ADC_0 1
 #endif
   #define HAS_TEMP_ADC_CHAMBER 1
 #endif
 
-#if HAS_HOTEND && EITHER(HAS_TEMP_ADC_0, HEATER_0_USES_MAX6675)
+#if HAS_HOTEND && ANY(HAS_TEMP_ADC_0, HEATER_0_USES_MAX6675, HEATER_0_DUMMY_THERMISTOR)
   #define HAS_TEMP_HOTEND 1
 #endif
 #define HAS_TEMP_BED        HAS_TEMP_ADC_BED

Marlin/src/inc/SanityCheck.h

  */
 #if HEATER_0_USES_MAX6675 && !PIN_EXISTS(MAX6675_SS)
   #error "MAX6675_SS_PIN (required for TEMP_SENSOR_0) not defined for this board."
-#elif HAS_HOTEND && !HAS_TEMP_HOTEND
+#elif HAS_HOTEND && !HAS_TEMP_HOTEND && !HEATER_0_DUMMY_THERMISTOR
   #error "TEMP_0_PIN (required for TEMP_SENSOR_0) not defined for this board."
 #elif EITHER(HAS_MULTI_HOTEND, HEATERS_PARALLEL) && !HAS_HEATER_1
   #error "HEATER_1_PIN is not defined. TEMP_SENSOR_1 might not be set, or the board (not EEB / EEF?) doesn't define a pin."
     #error "MAX6675_SS2_PIN (required for TEMP_SENSOR_1) not defined for this board."
   #elif TEMP_SENSOR_1 == 0
     #error "TEMP_SENSOR_1 is required with 2 or more HOTENDS."
-  #elif !ANY_PIN(TEMP_1, MAX6675_SS2)
+  #elif !ANY_PIN(TEMP_1, MAX6675_SS2) && !HEATER_1_DUMMY_THERMISTOR
     #error "TEMP_1_PIN not defined for this board."
   #elif ENABLED(TEMP_SENSOR_1_AS_REDUNDANT)
     #error "HOTENDS must be 1 with TEMP_SENSOR_1_AS_REDUNDANT."
       #error "TEMP_SENSOR_2 is required with 3 or more HOTENDS."
     #elif !HAS_HEATER_2
       #error "HEATER_2_PIN not defined for this board."
-    #elif !PIN_EXISTS(TEMP_2)
+    #elif !PIN_EXISTS(TEMP_2) && !HEATER_2_DUMMY_THERMISTOR
       #error "TEMP_2_PIN not defined for this board."
     #endif
     #if HOTENDS > 3
         #error "TEMP_SENSOR_3 is required with 4 or more HOTENDS."
       #elif !HAS_HEATER_3
         #error "HEATER_3_PIN not defined for this board."
-      #elif !PIN_EXISTS(TEMP_3)
+      #elif !PIN_EXISTS(TEMP_3) && !HEATER_3_DUMMY_THERMISTOR
         #error "TEMP_3_PIN not defined for this board."
       #endif
       #if HOTENDS > 4
           #error "TEMP_SENSOR_4 is required with 5 or more HOTENDS."
         #elif !HAS_HEATER_4
           #error "HEATER_4_PIN not defined for this board."
-        #elif !PIN_EXISTS(TEMP_4)
+        #elif !PIN_EXISTS(TEMP_4) && !HEATER_4_DUMMY_THERMISTOR
           #error "TEMP_4_PIN not defined for this board."
         #endif
         #if HOTENDS > 5
             #error "TEMP_SENSOR_5 is required with 6 HOTENDS."
           #elif !HAS_HEATER_5
             #error "HEATER_5_PIN not defined for this board."
-          #elif !PIN_EXISTS(TEMP_5)
+          #elif !PIN_EXISTS(TEMP_5) && !HEATER_5_DUMMY_THERMISTOR
             #error "TEMP_5_PIN not defined for this board."
           #endif
           #if HOTENDS > 6
               #error "TEMP_SENSOR_6 is required with 6 HOTENDS."
             #elif !HAS_HEATER_6
               #error "HEATER_6_PIN not defined for this board."
-            #elif !PIN_EXISTS(TEMP_6)
+            #elif !PIN_EXISTS(TEMP_6) && !HEATER_6_DUMMY_THERMISTOR
               #error "TEMP_6_PIN not defined for this board."
             #endif
             #if HOTENDS > 7
                 #error "TEMP_SENSOR_7 is required with 7 HOTENDS."
               #elif !HAS_HEATER_7
                 #error "HEATER_7_PIN not defined for this board."
-              #elif !PIN_EXISTS(TEMP_7)
+              #elif !PIN_EXISTS(TEMP_7) && !HEATER_7_DUMMY_THERMISTOR
                 #error "TEMP_7_PIN not defined for this board."
               #endif
             #elif TEMP_SENSOR_7 != 0