r485 broke Python bindings

[artemp]

Starting in r485, e.g.

datasource_bmpolygons = PostGIS(host=conf.db_host,
                                dbname=conf.db_name,
                                user=conf.db_user,
                                password=conf.db_password,
                                table='bmpolygon_joined_current')

where conf.db_host and conf.db_password are both None, fails with

connection  [host= dbname=cycling user=cycling password= connect_timeout=4] failed
could not translate host name "dbname=cycling" to address: Name or service not known

[artemp]

[artem] Work fine if host and user are None :

>>> ds = PostGIS(host=None, dbname='openstreetmap', user=None,table='planet_osm_point')

but fails with empty strings:

>>> ds = PostGIS(host='', dbname='openstreetmap', user=None,table='planet_osm_point')

Fixed in r615