test: transformer empty line fixtures

[richardgill]

Description

Shiki's focus and highlight transformers remove the <span> entirely if the comment line is otherwise empty.

This PR adds a fixture which tests this behavior.

Linked Issues

This behavior is discussed in #589

Additional context

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.31%. Comparing base (be74312) to head (e2f1ea0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #636   +/-   ##
=======================================
  Coverage   96.31%   96.31%           
=======================================
  Files          70       70           
  Lines        5699     5699           
  Branches      755      756    +1     
=======================================
  Hits         5489     5489           
  Misses        205      205           
  Partials        5        5           

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