[ffigen] Don't generate native code for stub protocols

[liamappelbe]

Skip generating Objective-C code for stubbed protocols. This means that we don't need Objective-C code at all for the example app.

[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/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 92.07% (+0.1%) from 91.952%
when pulling d384057 on ffigen_proto_stub
into 70969fa on main.