fix removing key/value in map_column__remove query

[artemki2077]

when requesting map_column__remove, in the ModalQuerySet object, in the update function, when requesting a delete operation, the set object is converted to a dict with all values None, which causes an error when calling the to_database function

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• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[dkropachev]

@artemki2077 , thanks for your contribution, can I task you with providing a test for this bug.

[artemki2077]

@artemki2077 , thanks for your contribution, can I task you with providing a test for this bug.

I'm not a specialist in unit tests, but I can try to do it. But there is a bug there if you make any request to map_column__remove. I just copied this query from the documentation, at first I thought I had a mistake somewhere, but then I realized that the error was when calling to_database with the value None, when I added a check to it, everything worked.

[dkropachev]

@artemki2077 , there are set of tests on the functionality and they work just fine, please check it, probably this problem not in the driver.