obexd: Do not send empty vCard attributes
obexd: Do not send empty vCard attributes

PBAP specification v1.1, chap. 5.1.4.1 and 5.4.4.1 states that
non-mandatory attributes shall be sent only when their value is not NULL.
In case of vCards this can be simply understood as non-empty, therefore
the patch makes phonebook_add_contact add only mandatory attributes for
specified vCard version as well as those for which values are of non-zero
length.

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