Fix GitLab project import when a user has access only to their default namespace

What does this MR do?

It fixes a bug when a namespace ID was not passed to /import/gitlab_project/new page. It occurred when a user have no choice of the namespace, so we did not render the input for namespace ID. This MR introduces a hidden input for the described case.

Does this MR meet the acceptance criteria?

• 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?

Fixes #23507 (closed)