Fix wxNullVariant handling in wxDataViewBitmapRenderer

In both the generic and GTK+ implementations, setting the value of a
bitmap column to wxNullVariant resulted in the bitmap, if set for some
rows, being repeated on the rows with null value.

src/gtk/dataview.cpp

@@ -2432,7 +2432,7 @@ bool wxDataViewBitmapRenderer::SetValue( const wxVariant &value )
     }
     else
     {
-        return false;
+        SetPixbufProp(m_renderer, NULL);
     }
 
     return true;