Handle commits without descriptions for changelogs

When processing commits to include in a changelog, we may encounter
commits without a description (= they only have a subject). This results
in Commit#description being nil, resulting in a NoMethodError when we
try to extract a revert SHA from that description.

This commit fixes this by just not matching if the description is nil.
This is fine since commits without a description are never included, due
to a lack of a trailer tag.

This fixes https://gitlab.com/gitlab-org/gitlab/-/issues/323998

app/finders/repositories/changelog_commits_finder.rb

@@ -93,7 +93,7 @@ module Repositories
     end
 
     def revert_commit_sha(commit)
-      matches = commit.description.match(REVERT_REGEX)
+      matches = commit.description&.match(REVERT_REGEX)
 
       matches[:sha] if matches
     end

changelogs/unreleased/changelog-commits-without-description.yml

+---
+title: Handle commits without descriptions for changelogs
+merge_request: 56224
+author:
+type: fixed

spec/finders/repositories/changelog_commits_finder_spec.rb

@@ -64,4 +64,30 @@ RSpec.describe Repositories::ChangelogCommitsFinder do
       expect(commits.count).to eq(4)
     end
   end
+
+  describe '#revert_commit_sha' do
+    let(:finder) { described_class.new(project: project, from: 'a', to: 'b') }
+
+    it 'returns the SHA of a reverted commit' do
+      commit = double(
+        :commit,
+        description: 'This reverts commit 152c03af1b09f50fa4b567501032b106a3a81ff3.'
+      )
+
+      expect(finder.send(:revert_commit_sha, commit))
+        .to eq('152c03af1b09f50fa4b567501032b106a3a81ff3')
+    end
+
+    it 'returns nil when the commit is not a revert commit' do
+      commit = double(:commit, description: 'foo')
+
+      expect(finder.send(:revert_commit_sha, commit)).to be_nil
+    end
+
+    it 'returns nil when the commit has no description' do
+      commit = double(:commit, description: nil)
+
+      expect(finder.send(:revert_commit_sha, commit)).to be_nil
+    end
+  end
 end