This commit improves handling of missing channel signatures in the
database:
- Return nil from auth proof accessors instead of empty slices so that
missing signatures are stored as NULL in SQL.
- Update public channel checks to require signature length > 0, which
properly handles existing empty bytea values in the database.
- Add regression test covering empty v1 and v2 channel signatures to
prevent future issues.
Elle Mouton
6328c4d897