lint for default values null, empty strings or none

[mirpedrol]

Lint for null values in the nextflow_schema.json to avoid errors where nulls are converted to "None" by the schema tooling. nf-core/sarek#1116 (comment)

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 #2328 (c10c891) into dev (37c3d06) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev    #2328      +/-   ##
==========================================
- Coverage   72.86%   72.86%   -0.01%     
==========================================
  Files          78       78              
  Lines        8774     8777       +3     
==========================================
+ Hits         6393     6395       +2     
- Misses       2381     2382       +1     

Impacted Files	Coverage Δ
nf_core/schema.py	78.77% <66.66%> (-0.07%)	⬇️

[mashehu]

Nice!