[ONOS-4163] Provider side changes to support stateful PCE and PCECC

Change-Id: I0a57ed1d1e505a2e94921fd5f5d92426a105ad12
diff --git a/providers/pcep/tunnel/src/test/java/org/onosproject/provider/pcep/tunnel/impl/TunnelServiceAdapter.java b/providers/pcep/tunnel/src/test/java/org/onosproject/provider/pcep/tunnel/impl/TunnelServiceAdapter.java
index 10be5f5..8e31508 100644
--- a/providers/pcep/tunnel/src/test/java/org/onosproject/provider/pcep/tunnel/impl/TunnelServiceAdapter.java
+++ b/providers/pcep/tunnel/src/test/java/org/onosproject/provider/pcep/tunnel/impl/TunnelServiceAdapter.java
@@ -94,17 +94,17 @@
 
     @Override
     public Collection<TunnelSubscription> queryTunnelSubscription(ApplicationId consumerId) {
-        return null;
+        return Collections.emptySet();
     }
 
     @Override
     public Collection<Tunnel> queryTunnel(Tunnel.Type type) {
-        return null;
+        return Collections.emptySet();
     }
 
     @Override
     public Collection<Tunnel> queryTunnel(TunnelEndPoint src, TunnelEndPoint dst) {
-        return null;
+        return Collections.emptySet();
     }
 
     @Override