spa_sync_props: remove pool userprops by setting empty-string

People have noted there's no way to remove a pool userprop, only zero
it. Turns vdev userprops had a method, by setting empty-string. So this
makes pool userprops follow the same behaviour.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16887