Tweak handling of empty files when generating the test YAML

[ewels]

• Error instead of warning for md5s of empty files
• Don't skip adding the md5 to the test YAML if an empty file is found

⚠️ UNTESTED ⚠️ - would be fab if someone else could give this a spin and check that it works as intended 🙏🏻

Fixes #1376

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

[codecov]

Codecov Report

Merging #1442 (8e3f636) into dev (bde4731) will not change coverage.
The diff coverage is 40.00%.

@@           Coverage Diff           @@
##              dev    #1442   +/-   ##
=======================================
  Coverage   65.67%   65.67%           
=======================================
  Files          50       50           
  Lines        5844     5844           
=======================================
  Hits         3838     3838           
  Misses       2006     2006           

Impacted Files	Coverage Δ
nf_core/modules/lint/module_tests.py	71.42% <0.00%> (ø)
nf_core/modules/test_yml_builder.py	49.29% <66.66%> (ø)

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[JoseEspinosa]

I give it a try and works like a charm 🚀

[ewels]

Awesome, thanks @JoseEspinosa ! Always a bit of a relief when you write code blind 😅