Apply review fixes for keypaths separator handling

[Copilot]

Describe your changes
Address the review-thread feedback on keypaths() separator validation and test coverage. This updates the separator contract so None preserves the default "." behavior while "" remains invalid.

• Validation logic

  • Default separator only when separator is None
  • Reject empty-string separators explicitly with the existing ValueError

• Test coverage

  • Replace the implicit default-case test with an explicit separator=None assertion
  • Keep coverage focused on the reviewed behavior only

if separator is None:
    separator = "."
if not type_util.is_string(separator) or len(separator) == 0:
    raise ValueError("separator argument must be a (non-empty) string.")

Related issue
Addresses the review comments in the linked PR review thread.

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests for the proposed changes.
• I have run the tests and there are not errors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (9120d48) to head (c248664).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #585   +/-   ##
=======================================
  Coverage   98.32%   98.32%           
=======================================
  Files          64       64           
  Lines        2392     2393    +1     
=======================================
+ Hits         2352     2353    +1     
  Misses         40       40           

Flag	Coverage Δ
unittests	98.32% <100.00%> (+<0.01%)	⬆️

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