OpenZFS 9193 - bootcfg -C doesn't work

OpenZFS 9193 - bootcfg -C doesn't work

When given an empty string as a rootds value, bootcfg -C fails with
the error message 'could not set nextboot: '' is an invalid name'.
This should be allowed because it represents clearing the nextboot
configuration.

Authored by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/9193
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/504645d227
Closes #7230