When some Git clients attempt to fetch a project without a namespace
(e.g. project.git instead of group/project.git), a database with large
number of rows may encounter a statement timeout while attempting to
look up an empty namespace (`WHERE route.path IS NULL`) in
`GitAccess#ensure_project_on_push!`.

To avoid this, we now check that a valid namespace is available before
attempting to do more validation.

Closes https://gitlab.com/gitlab-org/gitlab/issues/199207