Don't fail linting if md5sum for empty files are found in a stub test

[mirpedrol]

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[mashehu]

Perfect! 🎉

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (e62d113) 75.04% compared to head (40ee3b2) 74.96%.
Report is 1 commits behind head on dev.

Files	Patch %	Lines
nf_core/modules/lint/module_tests.py	53.33%	7 Missing ⚠️
nf_core/subworkflows/lint/subworkflow_tests.py	53.33%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2571      +/-   ##
==========================================
- Coverage   75.04%   74.96%   -0.09%     
==========================================
  Files          85       85              
  Lines        9288     9306      +18     
==========================================
+ Hits         6970     6976       +6     
- Misses       2318     2330      +12     

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