Cleanup asserts

[btecu]

Use Assert.Null instead of Assert.Equal(null, ...).

• I've read the guidelines for contributing and seen the walkthrough
• The code builds and tests pass locally (also verified by our automated build checks)
• Code follows the same patterns and style as existing code in this repo

[SamMonoRT]

consider updating all instances in the repo
https://github.com/search?q=repo%3Adotnet%2Fefcore+%22Assert.Equal%28false%2C%22&type=code

https://github.com/search?q=repo%3Adotnet%2Fefcore+%22Assert.Equal%28true%2C%22&type=code

cc @maumar

[btecu]

consider updating all instances in the repo https://github.com/search?q=repo%3Adotnet%2Fefcore+%22Assert.Equal%28false%2C%22&type=code

https://github.com/search?q=repo%3Adotnet%2Fefcore+%22Assert.Equal%28true%2C%22&type=code

cc @maumar

It cannot be replaced in many cases, when the type is not boolean but instead is an object.

[maumar]

@btecu Thank you for the contribution!