Fix [#34023] r54146 breaks name-display of default "UI_UL_list"

draw_item *is* optional (it then uses default C function), even though there is not much sense to register a class without it, except for our default UI_UL_list!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54155 954f8c5b-7b00-dc11-b283-0030488c597c