hs-v3: Improve accessor semantic of client cached object

Add an inline helper function that indicates if the cached object contains a
decrypted descriptor or not.

The descriptor object is NULL if tor is unable to decrypt it (lacking client
authorization) and some actions need to be done only when we have a decrypted
object.

This improves code semantic.

Fixes #33458

Signed-off-by: David Goulet <dgoulet@torproject.org>