Implementation note: Elasticsearch doesn't support "null_value" for object fields, and does not support setting a string value (our null token) to an object field either, so I don't see how we can get the same kind of queries we had with the Lucene backend ("path.to.embedded:_null_") working.
Maybe we could just support the concept of "exists" queries, and not comply with the user-provided token? That would mean transforming queries such as "queryBuilder.keyword().onField("path.to.embedded").matching(null)" to "exists" queries in Elasticsearch.