webadmin: Add StringUtils.is{,Not}Empty

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

Conflicts:
	frontend/webadmin/modules/uicompat/src/main/java/org/ovirt/engine/ui/uicompat/external/StringUtils.java

Change-Id: Ia26668e73a42523f412b07ca3bcc8ca0578aa550
Bug-Url: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>
(cherry picked from commit 0200a000c630e5f083a27deb182ac7b9830fbd61)