In the following use case the transition can only be executed, if there is an assignee on that issue.
Add the condition or validator to the desired transition or create a new transition.
Choose Assignee and select is required
Please assign the issue to proceed.
Fields required
Fields required or changed
Require a non empty assignee.
This use case is valid for both conditions and validators. The only difference is that you can specify an additional error message when using a validator.