Missing values should be last by default in field sorts

Description

Currently they are last in the Elasticsearch backend (because we let it manage the defaults), but not in the Lucene backend, where they are wherever the default value for the field type is (empty string, zero, ...).

Let's be consistent and use the Elasticsearch approach of sorting documents without a value last.

Also, there are apparently missing tests, since we currently don't have any failure.

Environment

None

Assignee

Yoann Rodière

Reporter

Yoann Rodière

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Priority

Major
Configure