@TramaineM ,

You can run the below code in background script or fixed script,

var gr = new GlideRecord('sys_history_line');
gr.addEncodedQuery('u_user=NULL');
gr.query();
while (gr.next()){
    var users = new GlideRecord('sys_user');
    users.addQuery('name', gr.user_name);
    users.query();
    while(users.next()){
        gr.u_user = users.name;
        gr.updateWithReferences();
    }
    
}

Here i have used field as user name which is available in History table because created by was empty 

Note:  There is already a field called user which is reference to sys_user table which you can use aswell.

Please mark this comment as Correct Answer/Helpful if it helped you.

Regards,

Swathi Sarang