fix: crash when insert nil value into a dictionary (facebook#30066)

Summary:
This PR attempts to fix issue facebook#28278 and facebook#29525

On Crashlytics, the following error occurs in file `RCTWebSocketModule.m` at method `-[RCTWebSocketModule webSocket:didFailWithError:]` when a nil value is inserted into a dictionary as a value.

```
Fatal Exception: NSInvalidArgumentException
*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0]
```

This PR is following the suggestion of this: facebook#28278 (comment) and it replaces the values of any property if it is nil. In detail:
- it converts `error` to empty NSString if the original value is nil
- it converts `socketID` to a NSNumber object, which stores `-1` if the original value is nil

## Changelog

[iOS] [Fixed] - A crash in WebSocket module

Pull Request resolved: facebook#30066

Test Plan: We were not able to reproduce the crash, but the report itself provided enough information to find a solution for this issue.

Reviewed By: shergin

Differential Revision: D24241147

Pulled By: sammy-SC

fbshipit-source-id: d5d632b49ca77b5d8be8b9c32358bef68f17d30a

React/CoreModules/RCTWebSocketModule.mm

@@ -166,7 +166,9 @@ - (void)webSocket:(RCTSRWebSocket *)webSocket didFailWithError:(NSError *)error
   NSNumber *socketID = [webSocket reactTag];
   _contentHandlers[socketID] = nil;
   _sockets[socketID] = nil;
-  [self sendEventWithName:@"websocketFailed" body:@{@"message" : error.localizedDescription, @"id" : socketID}];
+  NSDictionary *body =
+      @{@"message" : error.localizedDescription ?: @"Undefined, error is nil", @"id" : socketID ?: @(-1)};
+  [self sendEventWithName:@"websocketFailed" body:body];
 }
 
 - (void)webSocket:(RCTSRWebSocket *)webSocket