Fix removing the username from the git repository URL for pull mirroring

What does this MR do?

Sets project.import_data.user to nil when username_only_import_url is given an URL like http://example.com (i.e., no username specified).

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

Why was this MR needed?

(Another) regression in pull mirroring support.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3588 (closed)