[fabric] Check for assigned event emitter before sending key/mouse events

Summary: This diff adds null checks to avoid sending key and/or mouse events before the event emitter was assigned to the component.

Test Plan:
* Run Zeratul with Fabric enabled
* Hover over buttons
* Use up/down arrows in composer on user tag selector

 https://pxl.cl/4jVCb

Reviewers: shawndempsey, #rn-desktop

Reviewed By: shawndempsey

Differential Revision: https://phabricator.intern.facebook.com/D53555261

Author: Nick Lefever

packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm

@@ -1634,7 +1634,7 @@ - (BOOL)handleKeyboardEvent:(NSEvent *)event {
   static const char kRCTViewKeyboardEventEmittedKey = 0;
   NSNumber *emitted = objc_getAssociatedObject(event, &kRCTViewKeyboardEventEmittedKey);
   BOOL alreadyEmitted = [emitted boolValue];
-  if (!alreadyEmitted) {
+  if (!alreadyEmitted && _eventEmitter) {
     if (event.type == NSEventTypeKeyDown) {
       _eventEmitter->onKeyDown(keyEvent);
     } else {
@@ -1664,6 +1664,10 @@ - (void)keyUp:(NSEvent *)event {
 #pragma mark - Mouse Events
 
 - (void)sendMouseEvent:(BOOL)isMouseOver {
+  if (!_eventEmitter) {
+    return;
+  }
+
   NSPoint locationInWindow = self.window.mouseLocationOutsideOfEventStream;
   NSPoint locationInView = [self convertPoint:locationInWindow fromView:nil];