Add a DiscreteResources sub type representing an empty set

This is for ONOS-4281

Change-Id: Ic4b6e2f00555681bb2af39dc1cf764d325ae8744

+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.store.resource.impl;
+
+import com.google.common.collect.ImmutableSet;
+import org.onosproject.net.resource.DiscreteResource;
+import org.onosproject.net.resource.DiscreteResourceId;
+
+import java.util.List;
+import java.util.Optional;
+import java.util.Set;
+
+/**
+ * Represents an empty set of discrete resource.
+ */
+final class EmptyDiscreteResources implements DiscreteResources {
+    static final DiscreteResources INSTANCE = new EmptyDiscreteResources();
+
+    // for serializer
+    private EmptyDiscreteResources() {}
+
+    @Override
+    public Optional<DiscreteResource> lookup(DiscreteResourceId id) {
+        return Optional.empty();
+    }
+
+    @Override
+    public DiscreteResources difference(DiscreteResources other) {
+        return this;
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return true;
+    }
+
+    @Override
+    public boolean containsAny(List<DiscreteResource> other) {
+        return false;
+    }
+
+    @Override
+    public DiscreteResources add(DiscreteResources other) {
+        return other;
+    }
+
+    @Override
+    public DiscreteResources remove(List<DiscreteResource> removed) {
+        return this;
+    }
+
+    @Override
+    public Set<DiscreteResource> values() {
+        return ImmutableSet.of();
+    }
+}