Force assignee only if blank

Hi, 

We have a trigger set up in our system to stop tickets from becoming unassigned in our system as this could lead to a poor customer experience. The trigger is attached. 

This trigger annoys my agents a lot though as if they respond to a ticket and they set the assignee to another group then the ticket will assign to them when they submit it and then they can assign to another group. In effect it takes two actions to do something that should take one. 

Does anyone know a way around this issue?

Thankyou