[settings] List Users-> Filter changed by can be set with empty value

[adam-stasiak]

When I go to List Users
And Set filter Changed By with empty value (yes it is possible to choose empty value from dropdown)
Then I click Search

Expected results

I am not able to choose empty value from dropdown. When value is not set -> search operation should be not permitted.

Actual results

I can set empty value and then application got an error from backend.

Screenshots

Nagranie.z.ekranu.2021-01-12.o.13.10.21.mov

How to reproduce the bug

1. Go to list users
2. add filter Changed by
3. set empty value from dropdown
4. click search

Environment

commit 0f731f2

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• [x ] I have checked the superset logs for python stacktraces and included it here as text if there are any.
• [x ] I have reproduced the issue with at least the latest released version of superset.
• [x ] I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

[stale]

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[rusackas]

This is likely fixed by now, and is pretty out of date if not. If people are still encountering this in current versions (3.x) please open a new Issue or a PR to address the problem.