code: @Nullable
      protected final SSLSession sslSession() {
          checkState(!sessionProtocol.isTls() || sslSession != null,
                     "sslSession must be set for a TLS-enabled protocol: %s", sessionProtocol);
          return sessionProtocol.isTls() ? sslSession : null;
      }
docs: Returns the {@link SSLSession} of the connection.
  
  @return the {@link SSLSession}, or {@code null} if the {@link SessionProtocol} is not TLS.