Check if OLD is set when migrating issue assignees

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/32439, which we ran into when deploying the latest 9.2 RC.

cc @vsizov

@selfup @timothyandrew This needs to be picked into 9-2-stable as otherwise users won't be able to create issues while the migration runs.

approved this merge request

enabled an automatic merge when the pipeline for cb4ee4b1 succeeds

mentioned in issue #32437 (closed)

merged

mentioned in commit 0b946a7b

mentioned in issue #31934 (closed)

Picked into 9-2-stable will go into v9.2.0-rc4

mentioned in commit b9fc5bf0

Thanks