Some imported projects have NULL values for `label_links.label_id` for
some reason. This caused the issue export CSV to fail because the
exporter attempted to sort an array containing string values and `nil`
values.

We should clean up the database entries
(https://gitlab.com/gitlab-org/gitlab/issues/197307) for this with a NOT
NULL constraint, but this may require some careful consideration. For
now, to make exports work again, we just reject any invalid label links.

Closes https://gitlab.com/gitlab-org/gitlab/issues/197300