Align serialEventRun() with main.cpp invocation.

Author: feilipu

src/variantHooks.cpp

@@ -99,10 +99,6 @@ void initVariant(void)
 
 /*-----------------------------------------------------------*/
 #if ( configUSE_IDLE_HOOK == 1 )
-
-void serialEventRun(void)
-	{}
-
 /*
  * Call the user defined loop() function from within the idle task.
  * This allows the application designer to add background functionality
@@ -114,7 +110,7 @@ void serialEventRun(void)
 void vApplicationIdleHook( void )
 {
 	loop();
-	serialEventRun();
+	if (serialEventRun) serialEventRun();
 }
 
 #endif /* configUSE_IDLE_HOOK == 1 */