NPE in  case of null_value creation with value as null

Hey,

as we tried to understand the missing filter
we came across a null pointer exception in creating a default null_value in the mapping.

The NPE is thrown if we try to set the "null_value" to null.

```
PUT /foo
{
  "mappings": {
    "bar": {
      "properties": {
        "exception": {
          "null_value": null,
          "type": "integer"
        }
      }
    }
  }
}
```
