When restricting the switcher popup to the current workspace, we
filter out running apps with an empty window list (namely: no open
windows on the current workspace). However we may end up with an
empty window list even when not restricting items to the current
workspace when all windows of a running app are associated with a
different application via the transient_for hint.
To fix this, just filter out items with an empty window list
unconditionally.
https://bugzilla.gnome.org/show_bug.cgi?id=722434
