Cosmos db provider: Implement owned type null comparison

[JoasE]

Comparison used to compare by checking on primary key properties, but this doesn't work for cosmos as it will filter out any document that doesn't contain a property used in a query condition. Compare by c["Prop"] = null instead. Don't compare document roots as those can't be null. Fixes: #24087

• [x] I've read the guidelines for contributing and seen the walkthrough
• [ ] I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• [x] The code builds and tests pass locally (also verified by our automated build checks)
• [x] Commit messages follow this format:

        Summary of the changes
        - Detail 1
        - Detail 2

        Fixes #bugnumber

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Code follows the same patterns and style as existing code in this repo