ONOS-3593 Fix NPE when selector or treatment is not assigned in FlowRule.Builder

Change-Id: I4dc1257ddcb67f43fadf16f56b517db84a74b83f
diff --git a/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java b/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java
index 1164e05..64ebcd9 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java
@@ -301,8 +301,8 @@
         private Integer priority;
         private DeviceId deviceId;
         private Integer tableId = 0;
-        private TrafficSelector selector;
-        private TrafficTreatment treatment;
+        private TrafficSelector selector = DefaultTrafficSelector.builder().build();
+        private TrafficTreatment treatment = DefaultTrafficTreatment.builder().build();
         private Integer timeout;
         private Boolean permanent;