feat: warn if loaded changelog is whitespace-only

[caarlos0]

warns users that their provided changelog file was whitespace-only

[codecov]

Codecov Report

Merging #2885 (ae5648e) into main (c42a2fd) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2885      +/-   ##
==========================================
+ Coverage   84.50%   84.51%   +0.01%     
==========================================
  Files         111      111              
  Lines        8865     8872       +7     
==========================================
+ Hits         7491     7498       +7     
  Misses       1105     1105              
  Partials      269      269              

Impacted Files	Coverage Δ
internal/pipe/changelog/changelog.go	90.66% <100.00%> (+0.26%)	⬆️

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