Test returning invalid value from GetValue() in dataview sample

Add an item with unspecified year and update the custom model GetValue()
to not return anything in this case to check, and confirm, that all
implementations handle this properly by simply not showing anything in
the cell in this case.

samples/dataview/mymodels.h

@@ -26,7 +26,7 @@ class MyMusicTreeModelNode
 public:
     MyMusicTreeModelNode( MyMusicTreeModelNode* parent,
                           const wxString &title, const wxString &artist,
-                          unsigned int year )
+                          int year )
     {
         m_parent = parent;