No address available is returned from the LocationSensor when the LocationSensor resolves a latitude and longitude that it cannot provide an address for. The Google database, does not have addresses for the middle of the desert, a cow pasture, or for all the addresses in cities in towns .
The default latitude/longitude of the LocationSensor is 0,0. 0 latitude and 0 longitude is a place in the Atlantic Ocean just west of the coast of Africa. The database does not have an address for that. I expect you see No address… because you Gps receiver has not triggered a LocationChanged event.
You might like to read more about the CurrentAddress block in the LocationSensor HOW TO: Parse a LocationSensor's Current Address by SteveJG