Apply Null Object pattern

Change-Id: I9b4d30114b22dcd32b228e4f17bb541beed4ebed
diff --git a/core/net/src/main/java/org/onosproject/net/flow/impl/FlowRuleManager.java b/core/net/src/main/java/org/onosproject/net/flow/impl/FlowRuleManager.java
index 40829be..82c1c43 100644
--- a/core/net/src/main/java/org/onosproject/net/flow/impl/FlowRuleManager.java
+++ b/core/net/src/main/java/org/onosproject/net/flow/impl/FlowRuleManager.java
@@ -614,7 +614,7 @@
         public synchronized void run() {
             if (stages.size() > 0) {
                 process(stages.remove(0));
-            } else if (!hasFailed && fops.callback() != null) {
+            } else if (!hasFailed) {
                 fops.callback().onSuccess(fops);
             }
         }
@@ -651,13 +651,10 @@
                 operationsService.execute(this);
             }
 
-            if (fops.callback() != null) {
-                final FlowRuleOperations.Builder failedOpsBuilder =
-                    FlowRuleOperations.builder();
-                failures.forEach(failedOpsBuilder::add);
+            FlowRuleOperations.Builder failedOpsBuilder = FlowRuleOperations.builder();
+            failures.forEach(failedOpsBuilder::add);
 
-                fops.callback().onError(failedOpsBuilder.build());
-            }
+            fops.callback().onError(failedOpsBuilder.build());
         }
     }