add support for no families or parents
* src/GrampsDb/_GrampsBSDDB.py: txn handling for get_handles
* src/GrampsDb/_GrampsDbBase.py: txn handling for get_handles
* src/GrampsDb/_GrampsGEDDB.py: txn handling for get_handles
* src/GrampsDb/_GrampsXMLDb.py: txn handling for get_handles
svn: r5707