Handle commits without descriptions for changelogs

What does this MR do?

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 #323998 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content