Handle null return from ExtendedSSLSession.getRequestedServerNames on Android SDK 27/28

[tsushanth]

Fixes #9523.

On Android SDK 27 and 28, ExtendedSSLSession.getRequestedServerNames() can return null instead of an empty list (upstream bug: google/conscrypt#977). This causes a NullPointerException in MockWebServer when recording requestedServerNames on a RecordedRequest.

Change: one-line null guard in Platform.getHandshakeServerNames — replace direct .mapNotNull call with (session.requestedServerNames ?: emptyList()).mapNotNull { ... }. Already catches UnsupportedOperationException in the same block, so the null case fits the same defensive pattern.

[tsushanth]

The two CI failures appear to be pre-existing flakiness unrelated to this change:

• jvm (11): FastFallbackTest > preferCallConnectionOverDeferredConnection failed with an IPv4/IPv6 connection-ordering race — the test asserts one connection type was preferred over another in a timing-sensitive scenario. The same job passes on main and the PR only touches Platform.kt (the null-check for getRequestedServerNames), which is on an entirely different code path.
• android (37.0, playstore_ps16k): also passes on main at head.

Happy to re-run CI if the maintainer can trigger it, or I can rebase onto a fresh main commit to get a clean run.