Adding emptyTreatment() and emptySelector()

Note: There was a change that adds a DROP action to
a traffic treatment if there are no other actions present.
To get a traffic treatment without the drop rule, use
emptyTreatment()

Change-Id: I1f23ed5e1fa7519eb94fcafa85facbad815d5e9c
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 499e9e7..7249ac0 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
@@ -27,6 +27,7 @@
 import org.onosproject.net.flow.instructions.Instruction;
 import org.onosproject.net.flow.instructions.Instructions;
 
+import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 
@@ -41,6 +42,9 @@
 
     private final boolean hasClear;
 
+    private static final DefaultTrafficTreatment EMPTY
+            = new DefaultTrafficTreatment(Collections.emptyList());
+
     /**
      * Creates a new traffic treatment from the specified list of instructions.
      *
@@ -99,6 +103,15 @@
     }
 
     /**
+     * Returns an empty traffic treatment.
+     *
+     * @return empty traffic treatment
+     */
+    public static TrafficTreatment emptyTreatment() {
+        return EMPTY;
+    }
+
+    /**
      * Returns a new traffic treatment builder primed to produce entities
      * patterned after the supplied treatment.
      *