How do I check for an empty "assigned user" field for bulding a rule?

Desired Behavior: An item of the type "test session" can only be moved to the state of "in testing" if the "assigned user" field is NOT empty

My current effort: I am trying to build a trigger which would check for an empty value in the "assigned user" My line of thinking was to create a logical field which would indicate if the field "assigned user" is empty. I was using the computation feature in this way: isEmpty("Assigned User",0) but the system complains of mixed types in the statement.

Integrity Admin Noobie here and would appreciate any help.

Thank You very much