make MultiMap#create(Linked|Ordered)Set return empty set instead of list

platform/util/src/com/intellij/util/containers/MultiMap.java

@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2016 JetBrains s.r.o.
- *
- * 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.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 
 package com.intellij.util.containers;
 
@@ -23,7 +9,8 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 import java.io.Serializable;
-import java.util.*;import java.util.HashMap;
+import java.util.*;
+import java.util.HashMap;
 
 /**
  * Consider to use factory methods {@link #createLinked()}, {@link #createSet()}, {@link #createSmart()}, {@link #create(TObjectHashingStrategy)} instead of override.
@@ -287,6 +274,12 @@ public class MultiMap<K, V> implements Serializable {
       protected Collection<V> createCollection() {
         return ContainerUtil.newLinkedHashSet();
       }
+
+      @NotNull
+      @Override
+      protected Collection<V> createEmptyCollection() {
+        return Collections.emptySet();
+      }
     };
   }
 
@@ -298,6 +291,12 @@ public class MultiMap<K, V> implements Serializable {
       protected Collection<V> createCollection() {
         return new OrderedSet<V>();
       }
+
+      @NotNull
+      @Override
+      protected Collection<V> createEmptyCollection() {
+        return Collections.emptySet();
+      }
     };
   }