Improve null safety by returning an empty list

Change-Id: I88e673b1ed0b9edfe86c9b1b4004ad9b95271d29
diff --git a/core/store/dist/src/main/java/org/onosproject/store/intent/impl/GossipIntentStore.java b/core/store/dist/src/main/java/org/onosproject/store/intent/impl/GossipIntentStore.java
index 77dbff1..6360cc8 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/intent/impl/GossipIntentStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/intent/impl/GossipIntentStore.java
@@ -166,7 +166,7 @@
         if (data != null) {
             return data.installables();
         }
-        return null;
+        return ImmutableList.of();
     }