Remove 'Loading weather ...' placeholder text.

When working offline, I rather would have a loading icon instead of 'Loading weather ...' info text.

src/extension.js

@@ -1146,7 +1146,7 @@ const OpenweatherMenuButton = new Lang.Class({
     },
 
     rebuildCurrentWeatherUi: function() {
-        this._weatherInfo.text = _('Loading current weather ...');
+        this._weatherInfo.text = _(' ');
         this._weatherIcon.icon_name = 'view-refresh' + this.getIconType();
 
         this.destroyCurrentWeather();