Creating an index with alias fails unless options are specified

[Radiergummi]

This happens due to the way PHP serializes empty arrays. Elasticsearch expects a request like the following (taken from the docs):

PUT /test
{
  "aliases": {
    "alias_1": {},
    "alias_2": {
      "filter": {
        "term": { "user.id": "kimchy" }
      },
      "routing": "shard-1"
    }
  }
}

The library allows expressing the above fluently:

$connection->createIndex('test', fn(Index $index) => $index
    ->alias('alias_1')
    ->alias('alias_2', [
        'filter' => [
            'term' => [ 'user.id' => 'kimchy' ]
        ]
    ])
    ->alias('routing', 'shard-1')
);

But for the first case, we set an empty array as the default value (as the options are omitted from the call). Now, PHP doesn't know whether an empty array should be serialized to an object or an array, and (somewhat unfortunately) defaults to a JSON array, which triggers the following error:

Elasticsearch\Common\Exceptions\BadRequest400Exception 

  {"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"No alias is specified"}],"type":"illegal_argument_exception","reason":"No alias is specified"},"status":400}

  at vendor/elasticsearch/elasticsearch/src/Elasticsearch/Connections/Connection.php:675
    671▕             $exception = new ScriptLangNotSupportedException($responseBody. $statusCode);
    672▕         } elseif ($statusCode === 408) {
    673▕             $exception = new RequestTimeout408Exception($responseBody, $statusCode);
    674▕         } else {
  ➜ 675▕             $exception = new BadRequest400Exception($responseBody, $statusCode);
    676▕         }
    677▕ 
    678▕         $this->logRequestFail($request, $response, $exception);
    679▕ 

This can be alleviated by defaulting to an instance of \ArrayObject, instead.