Bulk: allow null values in action/metadata line parameters

Closes #11458

src/main/java/org/elasticsearch/action/bulk/BulkRequest.java

@@ -332,7 +332,7 @@ public class BulkRequest extends ActionRequest<BulkRequest> implements Composite
                             } else {
                                 throw new IllegalArgumentException("Action/metadata line [" + line + "] contains an unknown parameter [" + currentFieldName + "]");
                             }
-                        } else {
+                        } else if (token != XContentParser.Token.VALUE_NULL) {
                             throw new IllegalArgumentException("Malformed action/metadata line [" + line + "], expected a simple value for field [" + currentFieldName + "] but found [" + token + "]");
                         }
                     }

src/test/java/org/elasticsearch/action/bulk/BulkRequestTests.java

@@ -177,4 +177,12 @@ public class BulkRequestTests extends ElasticsearchTestCase {
                     e.getMessage().contains("Malformed action/metadata line [3], expected START_OBJECT or END_OBJECT but found [START_ARRAY]"), equalTo(true));
         }
     }
+
+    @Test
+    public void testSimpleBulk10() throws Exception {
+        String bulkAction = copyToStringFromClasspath("/org/elasticsearch/action/bulk/simple-bulk10.json");
+        BulkRequest bulkRequest = new BulkRequest();
+        bulkRequest.add(bulkAction.getBytes(Charsets.UTF_8), 0, bulkAction.length(), null, null);
+        assertThat(bulkRequest.numberOfActions(), equalTo(9));
+    }
 }

src/test/java/org/elasticsearch/action/bulk/simple-bulk10.json

@@ -0,0 +1,15 @@
+{ "index"  : {"_index":null, "_type":"type1", "_id":"0"} }
+{ "field1" : "value1" }
+{ "index"  : {"_index":"test", "_type":null, "_id":"0"} }
+{ "field1" : "value1" }
+{ "index"  : {"_index":"test", "_type":"type1", "_id":null} }
+{ "field1" : "value1" }
+{ "delete"  : {"_index":null, "_type":"type1", "_id":"0"} }
+{ "delete"  : {"_index":"test", "_type":null, "_id":"0"} }
+{ "delete"  : {"_index":"test", "_type":"type1", "_id":null} }
+{ "create"  : {"_index":null, "_type":"type1", "_id":"0"} }
+{ "field1" : "value1" }
+{ "create"  : {"_index":"test", "_type":null, "_id":"0"} }
+{ "field1" : "value1" }
+{ "create"  : {"_index":"test", "_type":"type1", "_id":null} }
+{ "field1" : "value1" }