Fixed source projects not filtering

What does this MR do?

Fixes the source project dropdown throwing an error when filtering. This was because it was trying to remotely filter by sending a request to a undefined URL. This fixes that by not remotely filtering when no URL exists.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52728

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