[WebSocket] Reason can be null which causes an exception with NSDictionary in websocketClosed

Summary:
I am not 100% sure what causes reason to be null but it does happen.
Closes https://github.com/facebook/react-native/pull/1483
Github Author: Stanislav Vishnevskiy <vishnevskiy@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/WebSocket/RCTWebSocketManager.m

@@ -107,7 +107,7 @@ RCT_EXPORT_METHOD(close:(NSNumber *)socketID)
 {
   [_bridge.eventDispatcher sendDeviceEventWithName:@"websocketClosed" body:@{
     @"code": @(code),
-    @"reason": reason,
+    @"reason": reason ? reason : [NSNull null],
     @"clean": @(wasClean),
     @"id": webSocket.reactTag
   }];