👌 IMPROVE: DocutilsRenderer.create_highlighted_code_block

[chrisjsewell]

Minor improvement, to handle null lexer_name,
and allow for the use of an alternative node class.

[codecov]

Codecov Report

Merging #488 (d380224) into master (885651f) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
- Coverage   90.03%   90.03%   -0.01%     
==========================================
  Files          16       16              
  Lines        2058     2057       -1     
==========================================
- Hits         1853     1852       -1     
  Misses        205      205              

Flag	Coverage Δ
pytests	90.03% <66.66%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
myst_parser/docutils_renderer.py	92.15% <66.66%> (-0.02%)	⬇️

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