optimize empty cached filters to return null

modules/elasticsearch/src/main/java/org/elasticsearch/index/cache/filter/support/AbstractConcurrentMapFilterCache.java

@@ -171,7 +171,7 @@ public abstract class AbstractConcurrentMapFilterCache extends AbstractIndexComp
             if (prev != null) {
                 docSet = prev;
             }
-            return docSet;
+            return docSet == DocSet.EMPTY_DOC_SET ? null : docSet;
         }
 
         public String toString() {

modules/elasticsearch/src/main/java/org/elasticsearch/index/cache/filter/support/AbstractWeightedFilterCache.java

@@ -180,7 +180,7 @@ public abstract class AbstractWeightedFilterCache extends AbstractIndexComponent
                 innerCache.putIfAbsent(cacheKey, cacheValue);
             }
 
-            return cacheValue.value();
+            return cacheValue.value() == DocSet.EMPTY_DOC_SET ? null : cacheValue.value();
         }
 
         public String toString() {

modules/elasticsearch/src/main/java/org/elasticsearch/index/cache/filter/support/FilterCacheHelper.java

@@ -1,23 +0,0 @@
-/*
- * Licensed to Elastic Search and Shay Banon under one
- * or more contributor license agreements.  See the NOTICE file
- * distributed with this work for additional information
- * regarding copyright ownership. Elastic Search licenses this
- * file to you 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.elasticsearch.index.cache.filter.support;
-
-public class FilterCacheHelper {
-}