[native_assets_cli] Make nullable fields required in syntax

[dcharkes]

The syntax setup methods remove fields from the underlying json if the values are null. That could lead to accidentally unsetting a value if omitted, which might not be what is expected from the API. (One could expect values not passed are not set.)

To keep life consistent, also make the nullable fields for constructors required.

(There were only 2 syntax fields missing in our codebase, see diff. They were doing the right thing, but being explicit is good.)

Inspired by: #2116 (comment)

[github-actions]

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 85.966%. remained the same
when pulling 0d1282e on syntax-nullable-required
into ce2e40b on main.