Update RCTSRWebSocket.m (#29419)

Summary:
Only add Sec-WebSocket-Protocol header if has _requestedProtocols.

## Changelog

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[iOS] [Fixed] - when Sec-WebSocket-Protocol header is empty vaulue, IIS server will return error 502.

Pull Request resolved: #29419

Reviewed By: cpojer

Differential Revision: D22782867

Pulled By: PeteTheHeat

fbshipit-source-id: d588997a345929b0c11c554d4452e612a336901a

Libraries/WebSocket/RCTSRWebSocket.m

@@ -462,7 +462,7 @@ - (void)didConnect
 
   CFHTTPMessageSetHeaderFieldValue(request, CFSTR("Origin"), (__bridge CFStringRef)_url.RCTSR_origin);
 
-  if (_requestedProtocols) {
+  if (_requestedProtocols && _requestedProtocols.count > 0) {
     CFHTTPMessageSetHeaderFieldValue(request, CFSTR("Sec-WebSocket-Protocol"), (__bridge CFStringRef)[_requestedProtocols componentsJoinedByString:@", "]);
   }