hs-v3: Always generate the descriptor cookie

It won't be used if there are no authorized client configured. We do that so
we can easily support the addition of a client with a HUP signal which allow
us to avoid more complex code path to generate that cookie if we have at least
one client auth and we had none before.

Fixes #27995

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