cache: guard null fields in Entry.writeTo() to fix NPE (#8962)

[iVamsi]

Fix NullPointerException in Cache.Entry.writeTo() when an HTTPS response has a null handshake.

Fix

Changed if (url.isHttps) to if (url.isHttps && handshake != null) so the TLS block is skipped when handshake is absent. The response still succeeds. It just won't be served from cache on subsequent requests.

Tests added

Three regression tests in CacheTest:

• HTTPS response with null handshake does not crash writeTo().
• Multiple HTTPS requests with null handshake all succeed
• HTTPS response with null handshake is not served from cache

All verified to fail before the fix and pass after.

Fixes #8962

[yschimke]

I'm not sure if there is a different root cause, that https://example.com:80/ shouldn't match. But this change is an improvement, NPE from our code is bad.

[yschimke]

Actually httpsResponseWithNullHandshakeIsNotServedFromCache suggests it isn't blindly accepting.