allow NULL for 'resource-factory' and use the default factory then
allow NULL for 'resource-factory' and use the default factory then

lovebug356/gupnp,GNOME/gupnp,GNOME/gupnp,ZachGoldberg/gupnp-gi,ZachGoldberg/gupnp-gi,ZachGoldberg/gupnp-gi,GNOME/gupnp,GNOME/gupnp,lovebug356/gupnp