altTab: Don't return from _init() if there are no windows/apps

If a new SwitcherPopup is created and there are no windows or apps to
switch through found, instead of returning from _init(), still
initialize the SwitcherPopup and let the check in SwitcherPopup.show()
return false to terminate the popup.

In both cases, with or without the return statements,
WindowManager._startSwitcher() will call SwicherPopup.destroy(), which
will try to disconnect signal handlers, destroy actors etc. Now if the
constructor can't finish creating the popup, some of the functions
called from _onDestroy() will fail and throw errors.

One of those cases is when window-switcher is limited to the current
workspace, and a WindowCyclerPopup is initiated on an empty workspace.
Because this._highlight hasn't been created, _onDestroy() will fail when
trying to destroy the actor of this._highlight.

Also, the actor of this._switcherList will not get destroyed in case
show() returns because this._items is empty. For example, this will
happen when a new AppSwitcherPopup is initialized with at least 1
running app, but 0 windows on the active workspace.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/168

js/ui/switcherPopup.js

@@ -330,6 +330,11 @@ var SwitcherPopup = GObject.registerClass({
             GLib.source_remove(this._initialDelayTimeoutId);
         if (this._noModsTimeoutId != 0)
             GLib.source_remove(this._noModsTimeoutId);
+
+        // Make sure the SwitcherList is always destroyed, it may not be
+        // a child of the actor at this point.
+        if (this._switcherList)
+            this._switcherList.destroy();
     }
 
     _select(num) {