ES6 no longer allows tracer to be null

[dsuurlant]

Per elasticsearch/elasticsearch 6.0, it is no longer allowed to pass null to the setTracer function: https://github.com/elastic/elasticsearch-php/commit/0270c4f

However in this bundle, tracer is set to null in the older ManagerFactory (5.2.x-dev says it's ES6 compatible), as well as the IndexService in 6.0-dev (see https://github.com/ongr-io/ElasticsearchBundle/blob/6.0-dev/Service/IndexService.php#L53 and https://github.com/ongr-io/ElasticsearchBundle/blob/6.0-dev/DependencyInjection/Compiler/MappingPass.php#L44)

PHP: 7.3 Symfony: 3.4.30 ES (server): 6.7.1 ES (php client): 6.7.2 Elasticsearch-bundle: 5.2.x-dev