Problem description:
- given:
	- I claim the user task in tasklist
- Remove the assignee of the task in cockpit by editing it manually
 
- when:
	- I claim the task again in tasklist
 
- then:
	- I get an error that the task is already claimed with the following message: An error happend while claiming the task. :
 Task '52908e8a-0a31-11e7-b08f-024213c7a31e' is already claimed by someone else.
 
- I get an error that the task is already claimed with the following message: An error happend while claiming the task. :
Hints: 
Checking the act_hi_op_log table shows that the deletion of the assignee leads to setting the assignee to  "" instead of null.