Add unit tests for constraint_append not null constraints

[MrPowers]

Make sure the constraint_append function works well for NOT NULL constraints in the test suite.

See this blog post for more on NOT NULL constraints: https://delta.io/blog/2022-11-21-delta-lake-contraints-check/

[cs23andris]

I'd be happy to add these tests, can you assign it to me please?

[robertkossendey]

@cs23andris Great, assigned you!

[cs23andris]

Unfortunately, NOT NULL constraints are not checked in the current implementation of the constraint_append function, because they're not stored as delta table properties but with the table schema. I think it'd be nice to extend the constraint_append to check for the nullability of columns as well. Can I proceed with that in this issue?

[robertkossendey]

Sounds good and thank you @cs23andris, feel free to work on this! :)

[MrPowers]

Awesome, glad we have unit tests to check the edge cases. Thanks for working on this one @cs23andris.