fix: don't append empty header/footer to changelog

[muesli]

This will trim any superfluous newlines from the Changelog. In particular, if you don't have a ReleaseHeader set, the file will not start with two empty new-lines, but instead directly begin with ## Changelog.

[vercel]

@muesli is attempting to deploy a commit to the Goreleaser Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

Merging #1918 (10a9be2) into master (a6c4b7e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
- Coverage   81.73%   81.72%   -0.01%     
==========================================
  Files          73       73              
  Lines        3866     3864       -2     
==========================================
- Hits         3160     3158       -2     
  Misses        580      580              
  Partials      126      126              

Impacted Files	Coverage Δ
internal/pipe/changelog/changelog.go	92.72% <100.00%> (-0.13%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a6c4b7e...10a9be2. Read the comment docs.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.