Fix crash in RCTEventEmitter

Summary:
According to the crash, RCTEventEmitter is being asked to emit events before RN is set up {emoji:1f928}. Neither the bridge, nor bridgeless mode is ready to send events.

In this scenario, drop the events on the floor instead of crashing.
Changelog: [Internal]

Reviewed By: naftaly

Differential Revision: D24634701

fbshipit-source-id: 933e5dfd15e5ee7c2215489305c71de46e78a9e5

Author: Peter Argany

React/CoreModules/RCTEventDispatcher.mm

@@ -65,7 +65,7 @@ - (void)sendAppEventWithName:(NSString *)name body:(id)body
                     method:@"emit"
                       args:body ? @[ name, body ] : @[ name ]
                 completion:NULL];
-  } else {
+  } else if (_invokeJS) {
     _invokeJS(@"RCTNativeAppEventEmitter", @"emit", body ? @[ name, body ] : @[ name ]);
   }
 }
@@ -77,7 +77,7 @@ - (void)sendDeviceEventWithName:(NSString *)name body:(id)body
                     method:@"emit"
                       args:body ? @[ name, body ] : @[ name ]
                 completion:NULL];
-  } else {
+  } else if (_invokeJS) {
     _invokeJS(@"RCTDeviceEventEmitter", @"emit", body ? @[ name, body ] : @[ name ]);
   }
 }
@@ -198,7 +198,7 @@ - (void)dispatchEvent:(id<RCTEvent>)event
 {
   if (_bridge) {
     [_bridge enqueueJSCall:[[event class] moduleDotMethod] args:[event arguments]];
-  } else {
+  } else if (_invokeJSWithModuleDotMethod) {
     _invokeJSWithModuleDotMethod([[event class] moduleDotMethod], [event arguments]);
   }
 }