btif: GetItemAttributes can return no attributes

If there are no attributes available, it's possible to return zero
attributes from a GetItemAttributes request.

This is triggered by some carkits who are unhappy when they only get
titles back (the only required item) and ask for the rest of the
attributes (which we don't have).

Test: connect to honda carkit and get snoop logs to confirm
Bug: 36055995
Bug: 35956792
Change-Id: I5a31e206565d212e456111d8c6b542c7a1569e5a