/**
     * Returns a document containing just the _id of the changed document.
     * <p>
     * For unsharded collections this contains a single field, _id, with the
     * value of the _id of the document updated.  For sharded collections,
     * this will contain all the components of the shard key in order,
     * followed by the _id if the _id isn’t part of the shard key.
     * </p>
     *
     * @return the document key, or null if the event is not associated with a single document (e.g. a collection rename event)
     */