Some attributes don't have any values available. In those cases don't
make the possible_values entry visible.

Fixes: a40cd7ef ("platform/x86: think-lmi: Add WMI interface support on Lenovo platforms")
Signed-off-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Link: https://lore.kernel.org/r/20230320003221.561750-3-mpearson-lenovo@squebb.caReviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>