Fix backward Elasticsearch pagination from a null cursor

What does this MR do and why?

Fix backward Elasticsearch pagination from a null cursor

pagination_filter_for_null_cursor emitted the same filter in both directions, restricting matches to documents whose sort field is missing. That is right going forward, where only the remaining null-sort documents follow the cursor, but wrong going backward: Elasticsearch sorts nulls last, so every document that has a sort value precedes the cursor and was being excluded. Paging back out of the null bucket returned a short page, or nothing at all.

Reachable today on any nullable sort served from Elasticsearch through Search::Elastic::Pagination

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