Need script to assign issue to user who created branch or made pull request if no user assigned

[Eric Blumberg]

I need a script to do the following:

Check if ticket is assigned to a user - I have this piece

 -- If not, check that the user has the assignable user permission

----- If they do, assign it to the current user

if (issue.fields.assignee != null) {

//Set assignee here,

I am hoping there is something I can do in scriptrunner, and also to run it under the current user using script runner