5515 dataset user hold doesn't reject empty tags

Author: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@752fd8dabccac68d6d09f82f3bf3561e055e400b