Implement (derive) Debug for MaybeUnset

[stanislav-tkach]

In our project we have multiple structures that derive SerializeRow and we often want to print them, but as MaybeUnset doesn't implement Debug we are forced to manually implement it.

Pre-review checklist

• 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.

[github-actions]

cargo semver-checks found no API-breaking changes in this PR! 🎉🥳
Checked commit: a490744

[muzarski]

We will probably merge it after #1198 so @Lorak-mmk doesn't have to resolve the conflicts while working on it