[native_toolchain_c] check null instead of assert

[MichealReed]

Given that there are multiple paths to resolve a tool, we should not assert if null, but instead avoid parsing the path if null.

Partially fixes #2109. NDK now resolves, other tools may still have errors.

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

[coveralls]

coverage: 85.96% (-0.7%) from 86.693%
when pulling 369e973 on MichealReed:fix-tool-resolver-error
into 543902c on dart-lang:main.

[dcharkes]

If it starts with $HOME, and homeDir_ is null, shouldn't we simply return []; early?

[dcharkes]

I was wondering why this could ever be null anyway, it should not be.

#2112

[MichealReed]

Do you still want to null check this? or should we close it? Maybe an env issue could still cause null here.

[dcharkes]

Yeah it wouldn't hurt to check and return [] if there is no home. Then at least the code doesn't crash if the home path environment variable isn't set. 👍

[MichealReed]

Cool, should be all set now.

[dcharkes]

Thanks for your contributions @MichealReed ! 🙏