cache: reject HTTPS responses with null handshake before cache write (#8962)

[iVamsi]

Improve the fix for NullPointerException in Cache.Entry.writeTo() when an HTTPS response has a null handshake.

Problem with previous fix (#9345)

The previous fix guarded writeTo() with handshake != null, which prevented the NPE but wrote malformed cache entries to disk. The Entry(rawSource) read constructor still expects TLS data when url.isHttps, so these entries could never be read back — wasting disk I/O on entries that always result in a cache miss.

Fix

Added guards in Cache.put() and Cache.update() to reject HTTPS responses with null handshake before serialization, so malformed entries are never written to disk. The writeTo() guard is kept as defense-in-depth.

Tests updated

Three regression tests in CacheTest:

• HTTPS response with null handshake is not cached
• Multiple HTTPS requests with null handshake all succeed
• HTTPS response with null handshake is not served from cache

All tests now assert cache.writeSuccessCount() == 0 to verify no entries are written to disk.

Fixes #8962