feat: add msgpack_strip_nulls() to shrink blobs by dropping nil map keys

[khanaffan]

Summary

Adds a new SQL function msgpack_strip_nulls(mp) that recursively removes map keys whose value is nil, shrinking the encoded size — useful for compacting sparse wide records (e.g. many optional columns encoded via msgpack_object(...) with NULL for absent fields).

Semantics

• Recurses into nested maps, including maps nested inside array elements.
• Array elements are always preserved — even a literal null inside an array stays, and array length/order never changes. Only map keys are dropped.
• Scalars and non-map/array roots pass through unchanged.
• NULL input → NULL; malformed msgpack → SQL error (never a crash).

SELECT msgpack_to_json(
  msgpack_strip_nulls(msgpack_from_json('{"a":1,"b":null,"c":{"x":null,"y":2}}'))
);
-- {"a":1,"c":{"y":2}}

SELECT msgpack_to_json(msgpack_strip_nulls(msgpack_array(1, null, 3)));
-- [1,null,3]   (array elements untouched)

Implementation

Mirrors the mpMergePatch rebuild style added in the recent security fix: a recursive mpStripNulls() walks the source buffer, buffers filtered map pairs in a temp MpBuf (since the final pair count isn't known until nils are excluded) before writing the map header, and appends array elements directly into the output since array element count never changes. Map/array header counts are bounds-checked against the remaining buffer before use — same hardening pattern as the mpMergePatch fix.

Testing

• 9 new regression checks (9.1–9.9) in test_spec_p5_mutation.c: top-level and recursive nil-stripping, array-element preservation (including a nested map inside an array), all-nil map → {}, scalar passthrough, NULL → NULL, deep multi-level nesting, a size-shrink assertion, and a malformed-MAP32-count rejection check.
• Added msgpack_strip_nulls to the fuzz_msgpack.c harness so it's exercised by both fuzz_corpus (ctest) and ad-hoc libFuzzer runs going forward.
• Full ctest suite: 16/16 passing.
• 45s libFuzzer+ASan run across the whole harness (now including msgpack_strip_nulls): 102,261 execs, 0 crashes.

Docs

README updated with a new #### msgpack_strip_nulls(mp) section alongside the other mutation functions.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com