Sonar fixes

Rule: Use isEmpty() to check whether the collection is empty or not.

Change-Id: Ib5ab56b2263623b297db56c5cac2c2ee80e12725
diff --git a/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java b/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java
index 623ece1..1fd3935 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/DefaultTrafficTreatment.java
@@ -498,7 +498,7 @@
 
         @Override
         public TrafficTreatment build() {
-            if (deferred.size() == 0 && immediate.size() == 0
+            if (deferred.isEmpty() && immediate.isEmpty()
                     && table == null && !clear) {
                 immediate();
                 noAction();