BugFix: controlled vocab values validation

[ErykKul]

What this PR does / why we need it: Bug fix.

Which issue(s) this PR closes:

• Closes #11900

Special notes for your reviewer:

• see also the test; it illustrates how the bug can happen

Suggestions on how to test this:

• hard to reproduce the bug, the easiest way was to write a test for it and ensure no regression can happen.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

• no

Is there a release notes update needed for this change?:

• probably not? Maybe one liner: "bugfix: controlled vocab values validation when empty"?

Additional documentation: