webadmin: Add StringUtils.is{,Not}Empty

Methods added to the extrenal class until StringUtils is supported
normally.

Change-Id: Ia26668e73a42523f412b07ca3bcc8ca0578aa550
Related-To: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>