[jnigen] Fix crash on Kotlin wildcards

[HosseinYousefi]

Kotlin wildcards (*) would cause the code generator to crash. Now I represent them in the type arg list as null.

[github-actions]

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

[coveralls]

coverage: 87.723% (-0.005%) from 87.728%
when pulling 6f5a242 on fix-kotlin-type
into a7fb6ee on main.

[liamappelbe]

Would it make sense to add an AST element for wildcards, rather than using null to represent it? If wildcards are only permitted in this arguments field, you could add a KotlinTypeArgument class that can be either a wildcard or a KotlinTypeProjection. That would be more self documenting.

[HosseinYousefi]

I get this from a JSON and use json_serializable to autogenerate the deserialization. I could do a custom thing for this one class though.