feat: add isNull and isNotNull filters

[damianstasik]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

This PR contains two new filters: isNull and isNotNull.

Related issue: #1658

Approach

It uses the same UI as the rest of the fields.

TODOs before merging

• Add tests
• Add changes to documentation (guides, repository pages, in-code descriptions)
• A changelog entry is created automatically using the pull request title (do not manually add a changelog entry)

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[mtrezza]

Did you have a look at #1721?

We discussed how to distinguish the different types of null filters. A Parse Server PR is also related to this.