M109 EOL fix when TEMP_RESIDENCY_TIME is undefined; printer interface (pronterface) could not see target extruder heater temperature to continue print

Marlin/Marlin.pde

@@ -864,6 +864,8 @@ FORCE_INLINE void process_commands()
               {
                  SERIAL_PROTOCOLLN( "?" );
               }
+            #else
+              SERIAL_PROTOCOLLN("");
             #endif
             codenum = millis();
           }