doc/obex-api: Mark DatabaseIdentifier as optional
doc/obex-api: Mark DatabaseIdentifier as optional

There is no need to declare DatabaseIdentifier in case it is invalid
since by omitting it should indicate GetVersion is not supported.

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