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 (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?

What are the relevant issue numbers?

Closes #3588 (closed)

Edited by Nick Thomas