Patch for returning empty String if no ALPN extension was read

[kaoh]

The Java SSLEngine
defines three states: null, an empty response and a string for the application protocol. getApplicationProtocol returns just null or or value. If no ALPN is found an empty string must be returned.

This fixes issue #1003

[koh-osug]

The build system has configuration issues. It reports:

Error: /home/runner/work/conscrypt/conscrypt/common/src/test/java/org/conscrypt/java/security/cert/CertificateFactoryTest.java:799: warning: [JdkObsolete] Date has a bad API that leads to bugs; prefer java.time.Instant or LocalDate.

for the original unchanged code.

[kaoh]

Local builds are working but remotely the ubuntu task always fails with a timeout.