Make return value null-safe

Change-Id: Ia56fac56ef8c036328ef6804386c0f86bd157f8f
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 4b8d20d..3636f14 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
@@ -217,8 +217,8 @@
                 .map(ControllerNode::id)
                 .filter(node -> !Objects.equals(node, me))
                 .collect(Collectors.toList());
-        if (nodes.size() == 0) {
-            return null;
+        if (nodes.isEmpty()) {
+            return ImmutableList.of();
         }
         return ImmutableList.of(nodes.get(RandomUtils.nextInt(nodes.size())));
     }