Fix #361: OnDemandList: empty preload node caused preload processing to stop.
Fix #361: OnDemandList: empty preload node caused preload processing to stop.

In cases where the first/top preload node was visible but empty, the
second/bottom one was not being processed.  The top one had to be scrolled
beyond the buffer limit before the second one was processed.

dylans/dgrid,kfranqueiro/dgrid,kfranqueiro/dgrid,maier49/dgrid,maier49/dgrid,kfranqueiro/dgrid,dylans/dgrid,maier49/dgrid,dylans/dgrid