Mapper: Throw exception if null_value is set to null

The mapping parser should throw an exception if "null_value" is set to `null`.

Fixes #7273

```bash
PUT /foo
{
  "mappings": {
    "bar": {
      "properties": {
        "exception": {
          "null_value": null,
          "type": "integer"
        }
      }
    }
  }
}
```
```
{
   "error": "MapperParsingException[mapping [bar]]; nested: MapperParsingException[Property [null_value] cannot be null.]; ",
   "status": 400
}
```