Fix custom text handling when not in embed mode

[jwalgran]

Overview

When not in embed mode just searching for free text was raising an exception because contributors was None in filter_by_query_params.

Connects #1652
Connects #1672

Testing Instructions

• Check out develop
• Search for test and verify that an exception prevents the search from working
• Check out this branch, run the same search, and verify that it works.
• Verify that text search in embed mode still works

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[TaiWilkin]

Thanks for the quick fix! This is working as expected now

[jwalgran]

Thanks for the review.