EventResponder runFromYield shortcut

Add quick test at start of function to return if there are no events pending.

Do this before checking if in ISR and the like.  Cut timing down a lot in yield.

teensy4/EventResponder.h

@@ -170,6 +170,7 @@ public:
 	EventResponder * waitForEvent(EventResponder *list, int listsize, int timeout);
 
 	static void runFromYield() {
+		if (!firstYield) return;
 		// First, check if yield was called from an interrupt
 		// never call normal handler functions from any interrupt context
 		uint32_t ipsr;