getApplicationProtocol returns null instead of empty String

Im using Android 11 and the native Conscrypt implementation but this should affect also the JRE version.

The [Java SSLEngine](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/javax/net/ssl/SSLEngine.html#getApplicationProtocol())
defines three states: `null`, an empty response and a string for the application protocol. [getApplicationProtocol](https://github.com/google/conscrypt/blob/1cca353e3133fbd587848a75debaed6eeab0779c/common/src/main/java/org/conscrypt/NativeCrypto.java#L1382) returns just `null` or or value. which is consumed by
[ActiveSession](https://github.com/google/conscrypt/blob/1cca353e3133fbd587848a75debaed6eeab0779c/common/src/main/java/org/conscrypt/ActiveSession.java#L286).

I have a TLS 1.2 page not including an ALPN and the Conscrypt SSLEninge returns for `getApplicationProtocol` null instead of an empty string.

I have created a [test project](https://github.com/koh-osug/alpn-issue) demonstrating the issue.

## Fix:

Return an empty string.