Simplify null count checking in column equality comparator

[vyasr]

Description

This PR removes an extra code path used for checking the equality of the null count when verifying if columns are equivalent (not equal). The purpose of this code path was to verify a specific definition of equivalence for columns containing unsanitized nulls, i.e. by ignoring the stored null count and directly verifying the validity of the underlying null mask. This is no longer necessary because we required sanitized null masks to be output from all libcudf APIs now (see the "libcudf expects nested types to have sanitized null masks" section in the developer guide), and this requirement will be enforced with the merge of #14559.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

This looks to be safe. As discussed offline with @nvdbaranec, this special function for null counting is to handle non-empty nulls, which we no longer support. The remaining todo here is to add a test with an EXPECT_FAIL for a column containing non-empty nulls to ensure that our assertion functions like expect_columns_equivalent will fail when provided such a column, ensuring that we won't accidentally have tests passing on such inputs in the future.

EDIT: This testing is now explicitly handled by those assertion functions as of #14559

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[vyasr]

/merge

[GregoryKimball]

@vyasr do you expect a performance impact from this simplification?

[GregoryKimball]

@vyasr do you expect a performance impact from this simplification?

No, this is only a test change

[vyasr]

Confirming the last comment, which I discussed with Greg offline.