Change AsyncSSLSocket getSecurityProtocol to handle unencrypted mode
Change AsyncSSLSocket getSecurityProtocol to handle unencrypted mode

Summary: Return empty string for `getSecurityProtocol` if the socket is good and the handshake failed, otherwise "TLS"

Reviewed By: knekritz

Differential Revision: D5647637

fbshipit-source-id: b95cbf39e4bb7f89b1ebcbc0238c2becba7ad42a

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