Sonar cleanup for PolicyEngineUtils

Sonar cleanup for PolicyEngineUtils

Changes to resolve some sonar "major" issues in the PolicyEngineUtils
module.

Patch 2: Updated the NotificationStoreTest junit to account for the return
of a "Collections.emptyList()" rather than "null" in StdPDPNotification
class.

Patch 3: Updated the StdPDPNotificationTest junit to account for the
return of a "Collections.emptyList()" rather than "null" in
StdPDPNotifaction class.

Issue-ID: POLICY-474
Change-Id: I925774ac181fd38d1206637f47ab27ba499629ef
Signed-off-by: Magnusen, Drew (dm741q) <dm741q@att.com>