Don't warn if side-effecting main hook returns nil

[frenchy64]

If I understand correctly, the main hook's return value is ignored.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.59%. Comparing base (65be913) to head (ff20a23).

Files with missing lines	Patch %	Lines
src/kaocha/plugin.clj	0.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #452      +/-   ##
==========================================
- Coverage   74.62%   74.59%   -0.04%     
==========================================
  Files          52       52              
  Lines        2822     2822              
  Branches      284      284              
==========================================
- Hits         2106     2105       -1     
  Misses        521      521              
- Partials      195      196       +1     

Flag	Coverage Δ
integration	54.28% <0.00%> (-0.04%)	⬇️
unit	69.27% <0.00%> (-0.04%)	⬇️

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