tvOS: TV nav event emitter should check for bridge

Summary:
When running with the packager in the tvOS simulator, reloading from the packager hits an assert in `RCTEventEmitter`, causing a crash.  The solution is for `RCTTVNavigationEventEmitter` to check for the existence of the bridge before attempting to send an event.

Manual testing.

[IOS] [BUGFIX] [RCTTVNavigationEventEmitter.m] - Fix crash when reloading in tvOS
Closes https://github.com/facebook/react-native/pull/17797

Differential Revision: D7014975

Pulled By: hramos

fbshipit-source-id: 0bf766e87267ca8592ff0cc0b3cb4621a8e8f9b5

React/Modules/RCTTVNavigationEventEmitter.m

@@ -46,7 +46,9 @@ RCT_EXPORT_MODULE()
 
 - (void)handleTVNavigationEventNotification:(NSNotification *)notif
 {
-  [self sendEventWithName:TVNavigationEventName body:notif.object];
+  if (self.bridge) {
+    [self sendEventWithName:TVNavigationEventName body:notif.object];
+  }
 }
 
 @end