engine_page: valid.Empty() handling None

On update() we should validate None as well.

Change-Id: Ia0d9e94c719bac40d6adb4e4939e3c2cfcec6fe8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1019884
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>