Allow a home bump of 0 when homing Z with probe

Marlin/src/module/motion.cpp

   // When homing Z with probe respect probe clearance
   const float bump = axis_home_dir * (
     #if HOMING_Z_WITH_PROBE
-      (axis == Z_AXIS) ? max(Z_CLEARANCE_BETWEEN_PROBES, home_bump_mm(Z_AXIS)) :
+      (axis == Z_AXIS && Z_HOME_BUMP_MM) ? max(Z_CLEARANCE_BETWEEN_PROBES, home_bump_mm(Z_AXIS)) :
     #endif
     home_bump_mm(axis)
   );