don't use annoying and unneeded in C++ casts of NULL to "T *" in all other files neither

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58227 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/propgridiface.h

@@ -76,7 +76,7 @@ public:
     /** This constructor is required for NULL. */
     wxPGPropArgCls( int )
     {
-        m_ptr.property = (wxPGProperty*) NULL;
+        m_ptr.property = NULL;
         m_flags = IsProperty;
     }
     wxPGProperty* GetPtr( wxPropertyGridInterface* iface ) const;