Interface behavior issue on Android for SSLEngine.getApplicationProtocol()

Hi,

I was building an HTTP/2 server using netty on Android 11, which needs an ALPN implementation.

And I found that when the ClientHello did not contain an ALPN extension, the server would fail after SSL handshake finished.

The `ConscryptEngine.getApplicationProtocol()` returns null after SSL handshake finished, which behaves against the JDK document for SSLEngine, saying _"an empty String if application protocols values will not be used"_.