Fix bugs in wxEnumProperty introduced by r76562.

The compilation fixes introduced run-time fixes due to not account for the use
of NULL pointers (which are not accepted as choices cache but are accepted as
choices values, go figure).

Closes #16266.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76569 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775