No HEATER_0_PIN needed with 0 EXTRUDERS (#18827)

Marlin/src/inc/SanityCheck.h

 /**
  * Test Heater, Temp Sensor, and Extruder Pins
  */
-#if !HAS_HEATER_0
+#if !HAS_HEATER_0 && EXTRUDERS
   #error "HEATER_0_PIN not defined for this board."
 #elif !ANY_PIN(TEMP_0, MAX6675_SS)
   #error "TEMP_0_PIN not defined for this board."