Fixes UnboundLocalError crash when deck_row is None by returning an
empty list instead of falling through to a query with an unbound deck_id.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The guard condition was inverted: `if not deck_row:` entered the query
block when no deck was found, causing `deck_row[0]` to crash with
'NoneType' object is not subscriptable. Flipped to `if deck_row:`
so the query runs only when the deck exists.