fix: make --release-notes flags more deterministic

[caarlos0]

if the a file is given, but is empty, leave the changelog empty (although it will be warned).

this makes whitespace-only and empty files have the same behavior.

[codecov]

Codecov Report

Merging #2888 (7142f6c) into main (456f8aa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2888   +/-   ##
=======================================
  Coverage   84.51%   84.51%           
=======================================
  Files         111      111           
  Lines        8872     8872           
=======================================
  Hits         7498     7498           
  Misses       1105     1105           
  Partials      269      269           

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

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