Fix double ok in M603

Firmware/Marlin_main.cpp

@@ -9205,8 +9205,8 @@ void FlushSerialRequestResend()
 // Execution of a command from a SD card will not be confirmed.
 void ClearToSend()
 {
-    previous_millis_cmd = _millis();
-	if ((CMDBUFFER_CURRENT_TYPE == CMDBUFFER_CURRENT_TYPE_USB) || (CMDBUFFER_CURRENT_TYPE == CMDBUFFER_CURRENT_TYPE_USB_WITH_LINENR)) 
+	previous_millis_cmd = _millis();
+	if (buflen && ((CMDBUFFER_CURRENT_TYPE == CMDBUFFER_CURRENT_TYPE_USB) || (CMDBUFFER_CURRENT_TYPE == CMDBUFFER_CURRENT_TYPE_USB_WITH_LINENR)))
 		SERIAL_PROTOCOLLNRPGM(MSG_OK);
 }