PBAP: Change string search comparison from NULL to empty

This change compares the searchValue atribute to empty
string value rather than comparing with the NULL object
since the searchValue object itself is never NULL and
only the value becomes an empty string. Without this
patch DUT was returning no list on Vcard-listing request
by PBAP Client.

Change-Id: I20cdfc0246af3057ba8ee268dee3de1893810638