### What problem does this PR solve?
When _parse_doc_id_filter_with_metadata returns [], the empty list is
falsy so the WHERE id IN (...) clause was silently skipped, causing the
full dataset to be returned instead of an empty result.
Change `if doc_ids:` to `if doc_ids is not None:` in both get_list() and
get_by_kb_id() to distinguish between no filter (None) and a filter that
matched zero documents ([]).
Fixes#14962
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
2dbe3b8a62