My Jira was upgraded from 7.13.x to 8.5.x
// below post-function if loop returns false in 7.13.x
if(issue.assignee){} => false
// Post Jira upgrade, below post-function if loop returns true in 8.5.x
if(issue.assignee){} => true
on both versions, create screen assignee field is 'automatic'.
I have a post-function to update assignee when unassigned based on few field selection.