weather: Track whether a location was set

Setting GWeatherInfo:location to null helpfully doesn't mean
"no location", but "NYC". This obviously isn't what we want
to show users, so track the location validity separately and
consider it when updating the label shown to users.

https://bugzilla.gnome.org/show_bug.cgi?id=780252

js/ui/dateMenu.js

@@ -296,6 +296,9 @@ const WeatherSection = new Lang.Class({
     },
 
     _getLabelText: function() {
+        if (!this._weatherClient.hasLocation)
+            return _("Select a location…");
+
         if (this._weatherClient.loading)
             return _("Loading…");