Remove runtime warning when setting a NULL shared-dir
Remove runtime warning when setting a NULL shared-dir

Now that the webdav channel can cope with a NULL shared dir (by not
creating the webdav server), we no longer need to reject NULL shared
directories with a warning.

This fixes part of https://bugzilla.redhat.com/show_bug.cgi?id=1175721

freedesktop-unofficial-mirror/spice__spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,guodong/spice-gtk