[BUGFIX] server: Skip responses without user ids when indexing

[frascuchon]

Pull Request Template

Since the user_id is a nullable column, the search engine must tackle responses for those scenarios. This PR changes the search engine and skips responses when the user_id is None.

This can be tackled in a better way in the future once we have a clearer view of how to deal with deleted users.

Refs: #5070

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

Checklist

• I added relevant documentation
• follows the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

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The URL of the deployed environment for this PR is https://argilla-quickstart-pr-5093-ki24f765kq-no.a.run.app