Allow an empty prefix in format dict

[mwouts]

Closes #1144

[github-actions]

Thank you for making this pull request.

Did you know? You can try it on Binder: or .

Also, the version of Jupytext developed in this PR can be installed with pip:

HATCH_BUILD_HOOKS_ENABLE=true pip install git+https://github.com/mwouts/jupytext.git@fix_1144

(this requires nodejs, see more at Developing Jupytext)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.73%. Comparing base (29a979f) to head (840a49b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1238      +/-   ##
==========================================
- Coverage   97.76%   97.73%   -0.03%     
==========================================
  Files          29       29              
  Lines        4468     4468              
==========================================
- Hits         4368     4367       -1     
- Misses        100      101       +1     

Flag	Coverage Δ
external	75.05% <ø> (ø)
functional	88.52% <ø> (ø)
integration	77.28% <ø> (ø)
unit	66.56% <ø> (ø)

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