Throw a more meaningful message when no document is specified for indexing

Currently the a user forgets to send a document when indexing, they get a cryptic error that doesn't point to the actual cause:

```
∴ curl -s -XPUT localhost:9200/test/doc/1 | python -mjson.tool
{
    "error": "ElasticSearchParseException[Failed to derive xcontent from (offset=0, length=0): []]",
    "status": 400
}
```

This change makes the error message a bit more readable for someone who runs into this issue:

```
∴ curl -s -XPUT localhost:9200/test/doc/1 | python -mjson.tool
{
    "error": "MapperParsingException[failed to parse, document is empty]",
    "status": 400
}
```

The Exception originates from https://github.com/elasticsearch/elasticsearch/blob/ed09ba0a18ebe720db32b78e5146d4383525d014/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java#L456

I chose to make this a MapperParsingException to match the Exception that would normally get thrown in the case that a document didn't map correctly: https://github.com/elasticsearch/elasticsearch/blob/ed09ba0a18ebe720db32b78e5146d4383525d014/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java#L540