fix: dimension table search ignored when the dashboard has no active filters (getDimensionFilterWithSearch)

[nishantmonu51]

• On an explore with an expanded dimension table and no active filters, typing in the search box changes nothing: the list stays the same and no query is issued for the search text.
• Since the unified expression filter refactor (refac: unified expression filter component across explore, canvas, alerts and reports #9746), the dashboard's where filter is undefined when there are no active filters. getDimensionFilterWithSearch returned undefined in that case, so the search text was dropped and the dimension table query ran without a where clause. Any active filter on the dashboard masked the bug.
• Treat an undefined filter as an empty AND so the search clause is still appended. This also covers the TDD search, dimension table exports and pivot search, which use the same helper.
• Added unit tests for the undefined filter cases.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!