Fix cycle analytics plan stage when commits are missing

Takes into account when commits are nil so the app doesn't throw an exception and also removes them.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24836

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together