Provide a default for Z_PROBE_LOW_POINT

Marlin/src/inc/Conditionals_LCD.h

 #define HAS_BED_PROBE (ENABLED(FIX_MOUNTED_PROBE) || ENABLED(Z_PROBE_ALLEN_KEY) || HAS_Z_SERVO_PROBE || ENABLED(Z_PROBE_SLED) || ENABLED(SOLENOID_PROBE) || ENABLED(SENSORLESS_PROBING) || ENABLED(RACK_AND_PINION_PROBE))
 #define PROBE_SELECTED (HAS_BED_PROBE || ENABLED(PROBE_MANUALLY) || ENABLED(MESH_BED_LEVELING))
 
-#if !HAS_BED_PROBE
+#if HAS_BED_PROBE
+  #ifndef Z_PROBE_LOW_POINT
+    #define Z_PROBE_LOW_POINT -5
+  #endif
+  #if ENABLED(Z_PROBE_ALLEN_KEY)
+    #define PROBE_TRIGGERED_WHEN_STOWED_TEST // Extra test for Allen Key Probe
+  #endif
+#else
   // Clear probe pin settings when no probe is selected
   #undef Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN
   #undef Z_MIN_PROBE_ENDSTOP
-#elif ENABLED(Z_PROBE_ALLEN_KEY)
-  // Extra test for Allen Key Probe
-  #define PROBE_TRIGGERED_WHEN_STOWED_TEST
 #endif
 
 #define HOMING_Z_WITH_PROBE (HAS_BED_PROBE && Z_HOME_DIR < 0 && ENABLED(Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN))