[objective_c] Add NSNull

[liamappelbe]

Add bindings for NSNull, and add support for it in the converter functions. ObjC has a null equivalent, nil, but you're not allowed to put nil in collections like NSArray for some reason. So in they added NSNull instead (very good null safety design, definitely not a hack 🙄). Fixes #2629

Also, fix #2627. NSMutableDictionary.of should return an NSMutableDictionary.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.1.0	9.0.0-dev.0	9.0.0	✔️

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Changelog Entry ✔️

Package	Changed Files

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API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources
objective_c	_FinalizablePointer	internal.dart::_ObjCReference::new::_finalizable

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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

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Files
pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 82.297%. remained the same
when pulling a9c9fef on ns_null
into 4d1f149 on main.