Shortcut dont call HardwareSerial::processevents if nothing active

This cut down some timings in yield.

teensy4/HardwareSerial.h

 	void enableSerialEvents();
 	void disableSerialEvents();
 	static void processSerialEvents();
+	static uint8_t serial_event_handlers_active;
 
 	using Print::write; 
 	// Only overwrite some of the virtualWrite functions if we are going to optimize them over Print version
 	#else	
 	static SerialEventCheckingFunctionPointer serial_event_handler_checks[7];
 	#endif
-	static uint8_t serial_event_handlers_active;
 
 
 

teensy4/yield.cpp

 	if (running) return; // TODO: does this need to be atomic?
 	running = 1;
 
+
 	// USB Serail - Add hack to minimize impact...
 	if (usb_enable_serial_event_processing && Serial.available()) serialEvent();
 
 	// Current workaround until integrate with EventResponder.
-	HardwareSerial::processSerialEvents();
+	if (HardwareSerial::serial_event_handlers_active) HardwareSerial::processSerialEvents();
 
 	running = 0;
 	EventResponder::runFromYield();
+	
 };