fix(dart): preserve null for optional list properties

[schani]

The bug

For Dart output with null safety (the default), an optional List<T>?
property round-tripped null as [] instead of preserving null:

factory Coordinate.fromJson(Map<String, dynamic> json) => Coordinate(
    onlyFor: json["onlyFor"] == null ? [] : List<String>.from(json["onlyFor"]!.map((x) => x)),
);

Map<String, dynamic> toJson() => {
    "onlyFor": onlyFor == null ? [] : List<dynamic>.from(onlyFor!.map((x) => x)),
};

Even though the field is declared nullable (List<String>?), the
(de)serialization logic silently collapsed null into [], losing the
null-vs-empty-list distinction that some APIs rely on semantically (e.g. a
consumer checking onlyFor == null to mean "not applicable" vs.
onlyFor == [] meaning "applicable to nothing").

Root cause

mapList in packages/quicktype-core/src/language/Dart/DartRenderer.ts
unconditionally substituted [] for a null list in both fromJson and
toJson, rather than following the same pattern mapClass already uses a
few lines below it for nullable class-typed properties
(x == null ? null : X.fromJson(x)).

The fix

One-line change: mapList now emits list == null ? null : ... instead
of list == null ? [] : ..., for both the deserialization and
serialization directions:

factory Coordinate.fromJson(Map<String, dynamic> json) => Coordinate(
    onlyFor: json["onlyFor"] == null ? null : List<String>.from(json["onlyFor"]!.map((x) => x)),
);

Map<String, dynamic> toJson() => {
    "onlyFor": onlyFor == null ? null : List<dynamic>.from(onlyFor!.map((x) => x)),
};

Scope is intentionally limited to List (arrays), matching the issue
title/report; Map handling in Dart wasn't touched.

Test coverage

test/languages.ts's Dart config already skipped
test/inputs/schema/optional-const-ref.schema with a comment describing
this exact bug:

/* Absent optional lists don't round-trip: the generated fromJson/toJson
   turn them into [], so the output no longer matches the input */
"optional-const-ref.schema",

That schema has an optional array property (coordinates) and a sample
(optional-const-ref.2.json) where the key is entirely absent. Dart's
fixture comparison already runs with allowMissingNull: true, which treats
an explicit null value as equivalent to an absent key
(test/lib/deepEquals.ts) — so once mapList emits null instead of
[], this fixture round-trips correctly. The skip entry (and its
explanatory comment) has been removed, re-enabling this fixture as
regression coverage.

Verification

• npm run build passes.
• npm run test:unit — 163 tests pass.
• Manually re-generated Dart from the issue's repro schema and from
  optional-const-ref.schema; confirmed null is now preserved through
  both fromJson and toJson instead of becoming [].
• Could not execute the Dart fixture end-to-end (FIXTURE=dart script/test)
  locally because the dart toolchain isn't available in this sandbox; CI
  will run the re-enabled optional-const-ref.schema fixture.

Fixes #2656

🤖 Generated with Claude Code