[gtk] Fixed CellRenderer.StartEditing_cb() to properly handle null gdk events

gtktreeview.c:gtk_tree_view_start_editing() will always call
_gtk_tree_view_column_cell_event() with a NULL event parameter,
which eventually gets passed along to gtk_cell_renderer_start_editing().

See https://bugzilla.xamarin.com/show_bug.cgi?id=10903 for details.