API: Catch empty commit messages

What does this MR do?

Catches empty commit messages as part of the API parameter validation.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50265, https://gitlab.com/gitlab-org/gitlab-ce/issues/50266, https://gitlab.com/gitlab-org/gitlab-ce/issues/50268

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides