Queries: Avoid NPE during query parsing

Workaround until Jackson is upgraded to 2.6.x.
Fixes #8629.

src/main/java/org/elasticsearch/common/xcontent/json/JsonXContentParser.java

@@ -88,6 +88,11 @@ public class JsonXContentParser extends AbstractXContentParser {
 
     @Override
     public BytesRef utf8Bytes() throws IOException {
+        // Tentative workaround for https://github.com/elastic/elasticsearch/issues/8629
+        // TODO: Remove this when we upgrade jackson to 2.6.x.
+        if (parser.getTextLength() == 0) {
+            return new BytesRef();
+        }
         return new BytesRef(CharBuffer.wrap(parser.getTextCharacters(), parser.getTextOffset(), parser.getTextLength()));
     }
 

src/test/java/org/elasticsearch/common/xcontent/cbor/CborXContentParserTests.java

@@ -0,0 +1,46 @@
+/*
+ * 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.common.xcontent.cbor;
+
+import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.xcontent.XContentFactory;
+import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.common.xcontent.XContentType;
+import org.elasticsearch.test.ElasticsearchTestCase;
+import org.junit.Test;
+
+import java.io.IOException;
+
+public class CborXContentParserTests extends ElasticsearchTestCase {
+
+    @Test
+    public void testEmptyValue() throws IOException {
+        BytesReference ref = XContentFactory.cborBuilder().startObject().field("field", "").endObject().bytes();
+
+        for (int i = 0; i < 2; i++) {
+            // Running this part twice triggers the issue.
+            // See https://github.com/elastic/elasticsearch/issues/8629
+            XContentParser parser = XContentFactory.xContent(XContentType.CBOR).createParser(ref);
+            while (parser.nextToken() != null) {
+                parser.utf8Bytes();
+            }
+        }
+    }
+}