`config('services.unsplash.access_key', '')` only falls back to '' when the key
is *absent* — if the env var is present but empty, config returns null, and
assigning null to the `string` property throws a TypeError, taking the whole
request down with a 500 during media search.
Cast to `(string)` so a null/empty key degrades gracefully to '' and the service
returns an empty result set (as it already does for a missing key) instead of
throwing.
Covered by a regression test for both services.
