Prevent PRs with empty diffs

[fthomas]

Closes: #2154

[codecov]

Codecov Report

Merging #2316 (85674a6) into master (dc98c0f) will decrease coverage by 0.04%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #2316      +/-   ##
==========================================
- Coverage   78.29%   78.24%   -0.05%     
==========================================
  Files         136      136              
  Lines        2331     2335       +4     
  Branches       55       58       +3     
==========================================
+ Hits         1825     1827       +2     
- Misses        506      508       +2     

Impacted Files	Coverage Δ
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0.00% <0.00%> (ø)
...n/scala/org/scalasteward/core/git/FileGitAlg.scala	98.30% <100.00%> (+0.02%)	⬆️
...in/scala/org/scalasteward/core/git/GenGitAlg.scala	92.59% <100.00%> (+0.28%)	⬆️

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