fix: default to all scopes in dataset search

[nusje2000]

Summary

Fix dataset search failing when no scopes are provided. When scope is an empty list, the search throws a 500 error. This defaults to all available scopes (title, description, tag, author) when none are specified.

Changes

• src/djehuty/web/database.py: Default to all scopes when search_query["scope"] is empty.

Approval Checklist

• I agree to follow Djehuty's code of conduct.
• I have read and I have follow the code contribution workflow.
• Code style and conventions were respected.
• Documentation has been updated where needed (README, docs, or examples).
• Review approved by at least one maintainer.
• Merge readiness (PR is squashed into a single commit and follows the commit template).

Issue Reference (optional - PRs may not be associated with an issue)

Extracted from #56 as requested in #56 (comment).

[gabrielakuhn]

Thank you very much @nusje2000! Please check the commit message template to create your squashed commit so it can be merged.