BluetoothStatus: hide the device separator if no devices are shown

When BluetoothApplet::show-full-menu property is notified (when you
switch from a disabled adapter / no adapter to an active one), we
would show all the menu, including the device separator, without
checking if any devices actually existed.

https://bugzilla.gnome.org/show_bug.cgi?id=637690

js/ui/status/bluetooth.js

@@ -267,7 +267,10 @@ Indicator.prototype = {
     _updateFullMenu: function() {
         if (this._applet.show_full_menu) {
             this._showAll(this._fullMenuItems);
+            if (this._hasDevices)
                 this._showAll(this._deviceItems);
+            else
+                this._deviceSep.hide();
         } else {
             this._hideAll(this._fullMenuItems);
             this._hideAll(this._deviceItems);