altTab: Don't error out if we don't have windows for an app

We are currently erroring out when the tab chain doesn't contain at
least one window for an app which might happen for windows that don't
take focus like xeyes. This leaves us in a state where we can't show
the switcher at all. Let's just ignore these apps instead of looking
broken.

js/ui/altTab.js

@@ -448,8 +448,6 @@ const AppSwitcher = new Lang.Class({
             });
             if (appIcon.cachedWindows.length > 0)
                 this._addIcon(appIcon);
-            else if (workspace == null)
-                throw new Error('%s appears to be running, but doesn\'t have any windows'.format(appIcon.app.get_name()));
         }
 
         this._curApp = -1;