Fix regression in Merge request form

What does this MR do?

Fixes regression causing an error message to appear when user first enters the Merge Request form.

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

The fix is in app/services/merge_requests/build_service.rb

Why was this MR needed?

To fix #24627 (closed)

Screenshots (if relevant)

Before: http://screencast.com/t/80tpcReDHie

After: http://screencast.com/t/CzsrgiaogR

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support 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

What are the relevant issue numbers?

Closes #24627 (closed)