Don't SELECT * just to check if a project exists

What does this MR do?

This adds a small optimisation to ProjectUrlConstrainer so we don't perform a SELECT * when we only care about whether or not a row is returned.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides