DistributedFlowRuleStore: getFlowEntries should never return null

resoves ONOS-478

[Merged from master]

Change-Id: I3fd84088afd9c9f616858037829cac70708cc66e
(cherry picked from commit 24f79eb3f3a97f268c04342f86018bf34d0e2e26)
diff --git a/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java b/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java
index ff85054..21f6128 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java
@@ -334,7 +334,7 @@
         } catch (IOException | TimeoutException | ExecutionException | InterruptedException e) {
             log.warn("Unable to fetch flow store contents from {}", replicaInfo.master().get());
         }
-        return null;
+        return Collections.emptyList();
     }
 
     private Set<FlowEntry> getFlowEntriesInternal(DeviceId deviceId) {