Fix null dereference in project_import_status

Discovered in https://gitlab.com/gitlab-org/release/tasks/-/issues/1300

This is a follow-up to !28915 (merged)

We had fixed the potential null reference problem for another part of this resource before, but since we've expanded on it since, it occurred again.

Fortunately, in !28915 (merged) I added a test suite for this resource, which we didn't have before, so I can actually regression-test this now.