🐛 FIX: Convert empty directive option to None

[chrisjsewell]

fixes #431

[codecov]

Codecov Report

Merging #452 (715a85a) into master (89ad170) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #452   +/-   ##
=======================================
  Coverage   90.80%   90.80%           
=======================================
  Files          16       16           
  Lines        1881     1881           
=======================================
  Hits         1708     1708           
  Misses        173      173           

Flag	Coverage Δ
pytests	90.80% <100.00%> (ø)

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Impacted Files	Coverage Δ
myst_parser/parse_directives.py	88.23% <100.00%> (ø)

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