Hi @kaleske
I think id_employee = 0 is selected to the automatic changes status. For example when you create new order and select bank wire then your status will be awaiting for transfer. And no one from employee should be assigned to this change - it happened after order is placed.
Example: 
And in the database:
Above examples are in fresh Presta installation where I have done some tests - and there is no problem.
My problem is with other installation where id_employee = 1 and employee with this id been deleted long time ago before. We didn't have this problem on Prestashop 1.7.5 but after upgrade - now we have.
Maybe the solution will be as you mention update database and replace id_employee = 1 (in my case this employee not exist any more) to the one of existing employee.