[Mapper] Using default=null for _timestamp field creates a index loss on restart

Step to reproduce:

* Create new index and type.

```
DELETE new_index
PUT new_index
 {
    "mappings": {
    	"power": {
	        "_timestamp" : {
	            "enabled" : true,
	            "default": null
	        }
	    }
    }
}
```

* Add a document

```
PUT new_index/power/1
{
    "foo": "bar"
}
```

* Restart cluster ... and **index is missing**...

```
GET new_index
```

Gives IndexMissingException

Closes #9223.

(cherry picked from commit e654a2c)
(cherry picked from commit aef3bc2)

src/main/java/org/elasticsearch/cluster/metadata/MappingMetaData.java

@@ -354,7 +354,7 @@ public class MappingMetaData {
                     path = fieldNode.toString();
                 } else if (fieldName.equals("format")) {
                     format = fieldNode.toString();
-                } else if (fieldName.equals("default")) {
+                } else if (fieldName.equals("default") && fieldNode != null) {
                     defaultTimestamp = fieldNode.toString();
                 }
             }

src/test/java/org/elasticsearch/index/mapper/timestamp/TimestampMappingTests.java

@@ -632,6 +632,22 @@ public class TimestampMappingTests extends ElasticsearchSingleNodeTest {
         assertThat(mergeResult.hasConflicts(), is(true));
     }
 
+    /**
+     * Test for issue #9223
+     */
+    @Test
+    public void testInitMappers() throws IOException {
+        String mapping = XContentFactory.jsonBuilder().startObject()
+                .startObject("type")
+                    .startObject("_timestamp")
+                        .field("enabled", true)
+                        .field("default", (String) null)
+                    .endObject()
+                .endObject().endObject().string();
+        // This was causing a NPE
+        new MappingMetaData(new CompressedString(mapping));
+    }
+
     @Test
     public void testMergePaths() throws Exception {
         String[] possiblePathValues = {"some_path", "anotherPath", null};