Resolve "Merge request tabs don't render when no commits available"

What does this MR do?

Adds discussion tab and commits tab with empty state to empty merge requests.

Are there points in the code the reviewer needs to double check?

We need to ensure we test the behavior for both a branch with no commits or cases where the branch does not exist (if deleted after the merge request was created).

Why was this MR needed?

Colorless green ideas sleep furiously

Screenshots (if relevant)

Before:

After (Discussion Tab):

After (Commits Tab):

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #26155 (closed) and #24556 (closed)