Restore backward compatibility for "attach" verb.

In r332361 and r333439, two new parameters were added to geli attach
verb using gctl_get_paraml, which requires the value to be present.
This would prevent old geli(8) binary from attaching geli(4) device
as they have no knowledge about the new parameters.

Restore backward compatibility by treating the absense of these two
values as seeing the default value supplied by userland.

PR: 232595
Reviewed by: oshogbo
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17680