ui: rename EventsArgs.Empty to EMPTY and make final

Make it final and rename to be a constant.
Solves 2 findbugs warnings of types:
UWF_UNWRITTEN_PUBLIC_OR_PROTECTED_FIELD
NM_FIELD_NAMING_CONVENTION

Change-Id: Iff1f2de7e8aaaeb7869864d35da1b4892ad807f5
Signed-off-by: Alissa Bonas <abonas@redhat.com>