Prevent module linting KeyError edge case.

[ewels]

Fixes #1321

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 #1439 (5c248c4) into dev (36ce758) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1439      +/-   ##
==========================================
- Coverage   65.67%   65.66%   -0.01%     
==========================================
  Files          50       50              
  Lines        5847     5846       -1     
==========================================
- Hits         3840     3839       -1     
  Misses       2007     2007              

Impacted Files	Coverage Δ
nf_core/modules/lint/main_nf.py	80.00% <100.00%> (-0.12%)	⬇️

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