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: https://github.com/facebook/react-native/pull/29419
Reviewed By: cpojer
Differential Revision: D22782867
Pulled By: PeteTheHeat
fbshipit-source-id: d588997a345929b0c11c554d4452e612a336901a
fd85b84a86