Upstream: https://we.phorge.it/T16064
As a follow-up to this task: T393268: New Column Trigger: When moving to a column, assign to the "current user" (user that is performing the move)
Current trigger rules don't allow setting any conditions for when to trigger.
And given that one of the needs is to do this on unassigned tasks.
Supporting a check whether the issue was previously unassigned will probably need to be a separate rule/condition, such as "Assign unassigned task to..."