Fix regression caused by r76974 (inserting first choice item to wxEnumProperty).

This is a modified way of handling insertion of the first choice item to wxEnumProperty. Previous method based on special value (wxNOT_FOUND) used to indicate empty collection caused regression in wxPGProperty::DeleteChoice(). Currently, the size of the collection is checked directly in wxPGProperty::InsertChoice().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76977 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775