If we find an existing suitcase folder in HGSuitcaseInventoryService.GetRootFolder(), then return that as a response rather than null.

Fixes a regression from commit 346644016c529db6ed63aa866c7cf91cec1b635f (Fri Nov 15 23:10:59 2013)
This is actually the reverse of zadark's patch in http://opensimulator.org/mantis/view.php?id=6969
But I'm assuming that was a mistake since clearly we should be returning this data - this was original behaviour