Fixed crash if event doesn't have body parameter (#23711)

Summary:
We need to check `body[@"target"]` wether equal to `nil`, if it is, return to prevent crash.

[iOS] [Fixed] - Fixed crash if event doesn't have body parameter
Pull Request resolved: https://github.com/facebook/react-native/pull/23711

Differential Revision: D14298543

Pulled By: cpojer

fbshipit-source-id: d5e8cd69438f323ae102e61618c0371a01bee347

React/Base/RCTEventDispatcher.m

@@ -86,10 +86,14 @@ RCT_EXPORT_MODULE()
       @"Event body dictionary must include a 'target' property containing a React tag");
   }
   
+  if (!body[@"target"]) {
+    return;
+  }
+
   name = RCTNormalizeInputEventName(name);
   [_bridge enqueueJSCall:@"RCTEventEmitter"
                   method:@"receiveEvent"
-                    args:body ? @[body[@"target"], name, body] : @[body[@"target"], name]
+                    args:@[body[@"target"], name, body]
               completion:NULL];
 }