781: handle None from fullmatch reference regex

[lindsay-stevens]

Closes #781

Why is this the best possible solution? Were any other approaches considered?

• this code branch is for is_pyxform_reference but it didn't handle the case where there is no match (looks like reference but has other characters) - in which case the iteration over outer_matches would fail and throw an error like "None has no group"
• added test coverage for relevant usages of is_pyxform_reference:
  • repeat_count, now with tests showing common usage pattern outcomes
  • seed param, which had alternate usages already (constant/single ref)

What are the regression risks?

Fixes a regression, and tries to avoid others of a similar nature by adding more tests.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments

[lognaturel]

Thanks!