https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-exists-query.html#find-docs-null-values shows how to find a document where the field contains
 or null. But how can I search for documents where the field doesn't exist at all on the document?
So if the document contains that field but it is
 or null, the search should treat that document as if the field exists (and not include it in the results).