top level filter not resulting in an actual filter is ignored

when parsing a filter, we use null to indicate that this filter should not match anything, the top level filter doesn't take it into account
fixes #3356

src/main/java/org/elasticsearch/search/query/FilterParseElement.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.search.query;
 
+import org.apache.lucene.search.Filter;
+import org.elasticsearch.common.lucene.search.Queries;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.search.SearchParseElement;
 import org.elasticsearch.search.internal.SearchContext;
@@ -30,6 +32,10 @@ public class FilterParseElement implements SearchParseElement {
 
     @Override
     public void parse(XContentParser parser, SearchContext context) throws Exception {
-        context.parsedFilter(context.queryParserService().parseInnerFilter(parser));
+        Filter filter = context.queryParserService().parseInnerFilter(parser);
+        if (filter == null) {
+            filter = Queries.MATCH_NO_FILTER;
+        }
+        context.parsedFilter(filter);
     }
 }