Added wxPropertyGrid::GetUnspecifiedValueText(). Use it instead of assuming that the unspecified value text is always an empty string.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62955 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/propgrid/propgrid.cpp

@@ -3629,6 +3629,12 @@ void wxPropertyGrid::CustomSetCursor( int type, bool override )
     m_curcursor = type;
 }
 
+wxString
+wxPropertyGrid::GetUnspecifiedValueText( int WXUNUSED(argFlags) ) const
+{
+    return wxEmptyString;
+}
+
 // -----------------------------------------------------------------------
 // wxPropertyGrid property selection, editor creation
 // -----------------------------------------------------------------------