Change PID dummy value to NAN

Fixes #17078

Marlin/src/module/configuration_store.cpp

       HOTEND_LOOP() {
         PIDCF_t pidcf = {
           #if DISABLED(PIDTEMP)
-            DUMMY_PID_VALUE, DUMMY_PID_VALUE, DUMMY_PID_VALUE,
-            DUMMY_PID_VALUE, DUMMY_PID_VALUE
+            NAN, NAN, NAN,
+            NAN, NAN
           #else
                          PID_PARAM(Kp, e),
             unscalePID_i(PID_PARAM(Ki, e)),
 
       const PID_t bed_pid = {
         #if DISABLED(PIDTEMPBED)
-          DUMMY_PID_VALUE, DUMMY_PID_VALUE, DUMMY_PID_VALUE
+          NAN, NAN, NAN
         #else
           // Store the unscaled PID values
           thermalManager.temp_bed.pid.Kp,
           PIDCF_t pidcf;
           EEPROM_READ(pidcf);
           #if ENABLED(PIDTEMP)
-            if (!validating && pidcf.Kp != DUMMY_PID_VALUE) {
+            if (!validating && !isnan(pidcf.Kp)) {
               // Scale PID values since EEPROM values are unscaled
               PID_PARAM(Kp, e) = pidcf.Kp;
               PID_PARAM(Ki, e) = scalePID_i(pidcf.Ki);
         PID_t pid;
         EEPROM_READ(pid);
         #if ENABLED(PIDTEMPBED)
-          if (!validating && pid.Kp != DUMMY_PID_VALUE) {
+          if (!validating && !isnan(pid.Kp)) {
             // Scale PID values since EEPROM values are unscaled
             thermalManager.temp_bed.pid.Kp = pid.Kp;
             thermalManager.temp_bed.pid.Ki = scalePID_i(pid.Ki);

Marlin/src/module/temperature.h

   typedef IF<(LPQ_MAX_LEN > 255), uint16_t, uint8_t>::type lpq_ptr_t;
 #endif
 
-#define DUMMY_PID_VALUE 3000.0f
-
 #if ENABLED(PIDTEMP)
   #define _PID_Kp(H) Temperature::temp_hotend[H].pid.Kp
   #define _PID_Ki(H) Temperature::temp_hotend[H].pid.Ki
     #define _PID_Kf(H) 0
   #endif
 #else
-  #define _PID_Kp(H) DUMMY_PID_VALUE
-  #define _PID_Ki(H) DUMMY_PID_VALUE
-  #define _PID_Kd(H) DUMMY_PID_VALUE
+  #define _PID_Kp(H) NAN
+  #define _PID_Ki(H) NAN
+  #define _PID_Kd(H) NAN
   #define _PID_Kc(H) 1
 #endif