Do not track named queries that are null

Adding a named query that is null can lead to a NullPointerException
when copying the named queries. This is due to an implementation detail
in QueryParseContent.copyNamedQueries. In particular, this method uses
com.google.common.collect.ImmutableMap.copyOf. A documented requirement
of ImmutableMap is that none of the entries have a null key nor null
value. Therefore, we should not add such queries to the namedQueries
map. This will not change any behavior since Map.get returns null if no
entry with the given key exists anyway.

Closes #12683

core/src/main/java/org/elasticsearch/index/query/QueryParseContext.java

@@ -182,7 +182,9 @@ public class QueryParseContext {
     }
 
     public void addNamedQuery(String name, Query query) {
-        namedQueries.put(name, query);
+        if (query != null) {
+            namedQueries.put(name, query);
+        }
     }
 
     public ImmutableMap<String, Query> copyNamedQueries() {

core/src/test/java/org/elasticsearch/index/query/CommonTermsQueryParserTest.java

@@ -0,0 +1,52 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.query;
+
+import org.elasticsearch.action.search.SearchResponse;
+import org.elasticsearch.test.ESSingleNodeTestCase;
+import org.junit.Test;
+
+import java.io.IOException;
+
+public class CommonTermsQueryParserTest extends ESSingleNodeTestCase {
+    @Test
+    public void testWhenParsedQueryIsNullNoNullPointerExceptionIsThrown() throws IOException {
+        final String index = "test-index";
+        final String type = "test-type";
+        client()
+                .admin()
+                .indices()
+                .prepareCreate(index)
+                .addMapping(type, "name", "type=string,analyzer=stop")
+                .execute()
+                .actionGet();
+        ensureGreen();
+
+        CommonTermsQueryBuilder commonTermsQueryBuilder =
+                new CommonTermsQueryBuilder("name", "the").queryName("query-name");
+
+        // the named query parses to null; we are testing this does not cause a NullPointerException
+        SearchResponse response =
+                client().prepareSearch(index).setTypes(type).setQuery(commonTermsQueryBuilder).execute().actionGet();
+
+        assertNotNull(response);
+        assertEquals(response.getHits().hits().length, 0);
+    }
+}