Hi,
You cannot directly search a blank (empty) string in IDM user search. but you can find all the blank field in this way.
employee Search attribute = Employee Number and explaining the use case for modify user search screen:
1) Navigate to the "Roles and Task>> Admin Task>>Modify AdminTask" and Search for Modify User admin task.
2) Go to the search tab and edit the "Default User Search" search screen.
3) Add EmployeeNumber field in the search on, search result and mark sortable check box to checked in search result area. click OK>>Select>>Submit.
4) Navigate to User>>ManageUser>>Modifyuser and search user LoginID=* string.
5) Click on the EmployeeNumber for user sorting and there you will find all the users with empty (Blank) employee Number.
Thanks,
Bherulal Dhakad