[xmonad] Combo fix ReleaseResources when no windows are available

Hello. I dug around some in X.L.Combo and noticed some strangeness, integrate cannot return empty lists (but integrate' can). The code expect it to return an empty list if there are no windows to display. Actually the function will never be called since emptyLayout is used instead of doLayout when there are no windows to display. All this results in that ReleaseResources is not sent to the underlying layouts. I fixed it by introducing emptyLayout but it is also possible to do it by using runLayout instead, but this seem cleaner. /Anders Engström