Restrict Assignee to Single Group Field, Only when group field is populated.

[David Harkins]

I know we can use permissions to restrict the assignee values based on a single group picker field.

But if this field is not populated, then all agents should be eligible to have the issue assigned to them.

When the group field is populated by a scriptrunner, it is unassigned so it would be most helpful if only the users in the group can have the issue assigned to them.