Give a dummy response to M105 with zero thermistors (#15568)

Marlin/src/core/language.h

@@ -168,7 +168,6 @@
 #define MSG_INVALID_E_STEPPER               "Invalid E stepper"
 #define MSG_E_STEPPER_NOT_SPECIFIED         "E stepper not specified"
 #define MSG_INVALID_SOLENOID                "Invalid solenoid"
-#define MSG_ERR_NO_THERMISTORS              "No thermistors - no temperature"
 #define MSG_M115_REPORT                     "FIRMWARE_NAME:Marlin " DETAILED_BUILD_VERSION " SOURCE_CODE_URL:" SOURCE_CODE_URL " PROTOCOL_VERSION:" PROTOCOL_VERSION " MACHINE_TYPE:" MACHINE_NAME " EXTRUDER_COUNT:" STRINGIFY(EXTRUDERS) " UUID:" MACHINE_UUID
 #define MSG_COUNT_X                         " Count X:"
 #define MSG_COUNT_A                         " Count A:"

Marlin/src/gcode/temperature/M105.cpp

@@ -39,7 +39,9 @@ void GcodeSuite::M105() {
       #endif
     );
   #else // !HAS_TEMP_SENSOR
-    SERIAL_ERROR_MSG(MSG_ERR_NO_THERMISTORS);
+    // Hosts such as printrun send M105 to check if firmware is responding.
+    SERIAL_ECHOPGM(MSG_OK);
+    SERIAL_ECHOPGM(" T:0");
   #endif
 
   SERIAL_EOL();