Fix FunstionScore Query random_score without seed bug

Hi,

Quick fix for a random_score without seed. When we want to execute match all query with random sorting

```php
$Query = new Elastica\Query();
$QueryFS  = new Elastica\Query\FunctionScore();
$QueryFS->setRandomScore();
$Query->setQuery($QueryFS);
```

```json
{
    "query": {
        "function_score": {
            "random_score": []
        }
    }
}
```

This will fail in Elasticsearch because of "random_score": []: 

```json
{
    "query": {
        "function_score": {
            "random_score": {}
        }
    }
}
```

Should be like this :)
Added a test with empty seed to check if it workd in ES.

Pawel