public SearchResultsBatchWorkSource(SearchManager searchManager,
transformer - - the function used to transform the list of Searchables returned from the query to the type required
by the batch, may also perform filtering, so it is safe for the apply method to return null, the apply
method should also accept null values.
gets the list of entities that form the batch. The list is retrieved by executing the query and converting the
search result to a list of entities, it is only then that the transformer is returned, so the number of entities
per batch as returned from this method may differ from the batch size, and may in fact be empty. An empty list
returned from this method does not connote that there are no more batches, rather @see hasMoreBatches