Only process pending objectives if there are any.

Change-Id: Ibb34dce32e0bfc55b317036130ead2c1babbaeac
diff --git a/core/net/src/main/java/org/onosproject/net/flowobjective/impl/FlowObjectiveManager.java b/core/net/src/main/java/org/onosproject/net/flowobjective/impl/FlowObjectiveManager.java
index 29e5857..5ecfb14 100644
--- a/core/net/src/main/java/org/onosproject/net/flowobjective/impl/FlowObjectiveManager.java
+++ b/core/net/src/main/java/org/onosproject/net/flowobjective/impl/FlowObjectiveManager.java
@@ -218,7 +218,8 @@
         }
 
         private void processPendingObjectives(DeviceId deviceId) {
-            pendingObjectives.get(deviceId).forEach(obj -> {
+            pendingObjectives.getOrDefault(deviceId,
+                                           Collections.emptySet()).forEach(obj -> {
                 if (obj instanceof NextObjective) {
                     getDevicePipeliner(deviceId)
                             .next(Collections.singletonList((NextObjective) obj));