return NULL from GetVoidPtr() for NULL variants instead of asserting (closes #9873)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58392 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/variant.h

@@ -321,6 +321,9 @@ public:
 
     /**
         Gets the void pointer value.
+
+        Notice that this method can be used for null objects (i.e. those for
+        which IsNull() returns @true) and will return @NULL for them.
     */
     void* GetVoidPtr() const;