return null instead of empty doc set

null can more easily be optimized in other filters

Author: kimchy

src/main/java/org/elasticsearch/index/search/child/HasChildFilter.java

@@ -147,7 +147,7 @@ public DocIdSet getDocIdSet(IndexReader reader) throws IOException {
             if (idReaderTypeCache != null) {
                 return new ParentDocSet(reader, collectedUids, idReaderTypeCache);
             } else {
-                return DocIdSet.EMPTY_DOCIDSET;
+                return null;
             }
         }
 

src/main/java/org/elasticsearch/index/search/child/HasParentFilter.java

@@ -113,7 +113,7 @@ public DocIdSet getDocIdSet(IndexReader reader) throws IOException {
             if (idReaderTypeCache != null) {
                 return new ChildrenDocSet(reader, parents, idReaderTypeCache);
             } else {
-                return DocIdSet.EMPTY_DOCIDSET;
+                return null;
             }
         }