Fix Blocker/Critical sonar issues

Fix Blocker/Critical sonar issues

Fix Blocker/Critical sonar issues in policy/drools-applications module
https://sonar.onap.org/component_issues?id=org.onap.policy.drools-applications%3Adrools-pdp-apps#resolved=false|severities=CRITICAL%2CMAJOR

Fixed 2 issues,
Used isEmpty() to check whether the collection is empty or not.
getActor returns a string, there's no need to call toString()

Issue-Id:POLICY-111
Change-Id: I645420ee370ceb5fca959f870e76db2390e19929
Signed-off-by: shashikanth.vh <shashikanth.vh@huawei.com>