/import status REST endpoint: fix null-unsafe access to import_state

What does this MR do?

Fixes #214302 (closed)

Note that I have not added a test for this, since I am working on an MR in parallel which adds a full test suite for this class already. It will make it easier to merge these changes if I instead add a test in this MR: !28915 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry Note: clarified on Slack, since this was only just introduced and since this will be in one of the next auto-deploys, no changelog entry is necessary.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

See above; will add a unit test in !28915 (merged)