Fix positive ABD size assertion in abd_verify().

Gang ABDs without childred are legal, and they do have zero size.
For other ABD types zero size doesn't have much sense and likely
not working correctly now.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:  Alexander Motin <mav@FreeBSD.org>
Sponsored by:   iXsystems, Inc.
Closes #14795