Schema - don't make default null for null values.

[ewels]

Closes #1074

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 #1077 (47c7a99) into dev (344ff29) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev    #1077      +/-   ##
==========================================
- Coverage   72.21%   72.20%   -0.01%     
==========================================
  Files          36       36              
  Lines        4545     4547       +2     
==========================================
+ Hits         3282     3283       +1     
- Misses       1263     1264       +1     

Impacted Files	Coverage Δ
nf_core/schema.py	85.35% <66.66%> (-0.20%)	⬇️

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[KevinMenden]

Nice, that was fast 👍

[ewels]

Yeah, figured that I should just get on and fix it while I was thinking about it..