L042: Fix internal error "Attempted to make a parent marker from multiple files"

[barrywhart]

Brief summary of the change made

Fixes #3623

Are there any other side effects of this change that we should be aware of?

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[codecov]

Codecov Report

Merging #3655 (bb32805) into main (9986aaf) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3655   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          176       176           
  Lines        13334     13335    +1     
=========================================
+ Hits         13334     13335    +1     

Impacted Files	Coverage Δ
src/sqlfluff/rules/L042.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9986aaf...bb32805. Read the comment docs.