obexd: Support empty filename in obc_transfer_get
obexd: Support empty filename in obc_transfer_get

Passing an empty string as a filename for obc_transfer_get will be
similar to passing a NULL filename. This means a temporary file will be
created to store the content of the transfer.

NULL and "" are not exactly equivalent though: in case of NULL the file
will be automatically removed immediately after being open, which means
that the transfer initiator should also open the file to prevent it from
being removed (to be used from the modules). In this case, the filename
will not be exposed in D-Bus.

On the other hand, if "" is given, the file will be removed only in case
of error. So after success the transfer initiator should decide whether
the file should be removed or not.

This change is convenient in order to expose the same API in D-Bus.

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