Prohibit null for resources field in Intent

Change-Id: I128c6e63ccccaf817e83ff1c440a731fb98b42f7
diff --git a/core/api/src/main/java/org/onosproject/net/intent/HostToHostIntent.java b/core/api/src/main/java/org/onosproject/net/intent/HostToHostIntent.java
index e78d78c..7e6eed8 100644
--- a/core/api/src/main/java/org/onosproject/net/intent/HostToHostIntent.java
+++ b/core/api/src/main/java/org/onosproject/net/intent/HostToHostIntent.java
@@ -26,6 +26,7 @@
 import org.onosproject.net.flow.TrafficTreatment;
 import org.onosproject.net.intent.constraint.LinkTypeConstraint;
 
+import java.util.Collections;
 import java.util.List;
 
 import static com.google.common.base.Preconditions.checkNotNull;
@@ -86,7 +87,7 @@
                             TrafficSelector selector,
                             TrafficTreatment treatment,
                             List<Constraint> constraints) {
-        super(appId, null, selector, treatment, constraints);
+        super(appId, Collections.emptyList(), selector, treatment, constraints);
 
         // TODO: consider whether the case one and two are same is allowed
         this.one = checkNotNull(one);