Cascading Parameter when no value for children in hierarchy

hello,

i have level1 , Level2 and Level 3 as cascading parameters of a hierarchy. 

i have a situation where if the selected client doesn't have level 2 and level 3 how can we hide/leave unselected and still view the report.

Now if i preview without entering the value for level 2 it says Please select a Value for the Parameter 'Level 2'.

i had even changed the parameter prop to null, allow blank but still doesn't work. is there any workaround. 

Please help, thank in advance!