Initialize wxPGChoices object properly in its copy ctor if source object has no data.

Internal data must be always initialized whether the source object contains choices data or not.

This is the backport of r78484.
See #16855.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78515 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/property.h

@@ -847,6 +847,10 @@ public:
             m_data = a.m_data;
             m_data->IncRef();
         }
+        else
+        {
+            Init();
+        }
     }
 
     /**