Add variant_strip_nulls expression

[bojana-db]

What changes were proposed in this pull request?

Adds the SQL function variant_strip_nulls(v[, includeArrays]), which recursively removes Variant null fields from objects (and, by default, Variant null elements from arrays) in a Variant value.

Details:

• Object path: a field whose value is a Variant null (the JSON null literal) is removed entirely; recursion walks nested objects and arrays at all levels;
• includeArrays (BOOLEAN, default true): when true, Variant null elements are also removed from arrays; when false, array null elements are kept, but null fields of objects nested anywhere (including inside arrays) are still stripped;
• Empty containers are preserved: an object/array emptied by stripping stays {} / []; the parent is never collapsed to NULL;
• Non-container inputs — scalars and a top-level Variant null (parse_json('null')) — are returned unchanged;
• NULL intolerant: any NULL argument returns NULL.

Why are the changes needed?

There is no built-in way to drop Variant null entries from a Variant; today users must convert it to another type (e.g. a map) and back.

Does this PR introduce any user-facing change?

Yes, a new SQL function (and Scala/Python functions API.

How was this patch tested?

Unit tests.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code with Claude Opus 4.8