unit/test-hfp: Fix parsing empty string test
unit/test-hfp: Fix parsing empty string test

Empty string shall be ignored by AT parser and no response shall be
sent. Therefore this test should close HFP connection just after empty
string is sent, otherwise test hungs.

pkarasev3/bluez,silent-snowman/bluez,mapfau/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,silent-snowman/bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez