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

folly/io/async/AsyncSSLSocket.h

@@ -282,6 +282,9 @@ class AsyncSSLSocket : public virtual AsyncSocket {
   std::string getApplicationProtocol() noexcept override;
 
   std::string getSecurityProtocol() const override {
+    if (sslState_ == STATE_UNENCRYPTED) {
+      return "";
+    }
     return "TLS";
   }