get_xml will attempt to retrieve the xml response from the obs api.
If response failes, a dummy, blank xml file will be returned. This
makes the helper functions more robust against api connection errors.
If query failes, empty data will be simply be returned to the client.
The client is responsible for any response error checking.
The 'response' property is added to the ObsApi. This holds the
full requests response from the last api query. The client can check
this property to test for query success when required.