Clean up a minor bug in #1653

[mgjarrett]

What is the change?

Change logic in CrossSectionGroupManager to handle the case where BlockCollection._validRepresentativeBlockTypes is None.

Why is the change being made?

This logic was introduced in PR #1653. The logic checks for the length of _validRepresentativeBlockTypes, assuming it is always a list. It did not consider the case where _validRepresentativeBlockTypes is None.

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Checklist

• [x] This PR has only one purpose or idea.
• [x] Tests have been added/updated to verify any new/changed code.

• [x] The code style follows good practices.
• [x] The commit message(s) follow good practices.

• [x] The release notes have been updated if necessary.
• [x] The documentation is still up-to-date in the doc folder.
• [x] The dependencies are still up-to-date in pyproject.toml.

[mgjarrett]

@john-science this PR addresses a corner case bug in PR #1653 that wasn't caught until after it was merged.