Useless when empty (4)
I believe the answer is:
void
I believe this is a double definition.
'useless' is the first definition.
(thesaurus)
'empty' is the second definition.
('void' can be similar in meaning to 'empty')
'when' is the link.
(Other definitions for void that I've seen before include "Vacant; not valid" , "absence of a suit" , "Empty space or legally invalid" , "Nullify" , "Empty space or vacuum" .)