`motion_supmitter` not required anymore

[rrenkert]

With OpenSlides/openslides-backend#3105 and OpenSlides/openslides-client#5324
the backend and client allow the motion submitter to be null (the accounts have been deleted). The client handles this case according to the description in its issue.
The autoupdate-service needs to allow this field to be empty.
There are failing actions in openslides-go:
https://github.com/OpenSlides/openslides-go/actions/runs/19069020531