[pigeon] Treat NSNull as null for non-null Flutter API returns

[Xelorium]

Description

FlutterStandardReader substitutes NSNull for a nil element when it decodes a list, so a null reply for a non-null return value reaches the generated Swift code as NSNull rather than as nil. The generated guard only checks listResponse[0] == nil, so NSNull falls through to the force cast below it and aborts the process:

} else if listResponse[0] == nil {
  completion(.failure(PigeonError(code: "null-error", ...)))
} else {
  let result = listResponse[0] as! AuthenticationChallengeResponse  // 💥

This crashes production apps through webview_flutter_wkwebview: the plugin clears its native instance manager in WebViewFlutterPlugin.tearDownProxyAPIRegistrar(), which iOS triggers on sceneDidDisconnect / applicationWillTerminate. A WKNavigationDelegate.didReceiveAuthenticationChallenge reply that is still in flight then carries an identifier the native instance manager can no longer resolve, the codec reader returns nil, and the app dies with:

Could not cast value of type 'NSNull' (0x...) to 'webview_flutter_wkwebview.AuthenticationChallengeResponse' (0x...).

The same shape was reported before for other types (URLRequestWrapper in flutter/flutter#162437), which is expected: the generator emits this guard for every non-null return value, so any unresolvable instance crashes instead of reporting an error.

This PR treats NSNull as a null reply, so the existing null-error path handles it.

Verification

• Added a Swift generator unit test asserting the generated guard covers NSNull.
• Added a native regression test in platform_tests that replies NSNull to a non-null Flutter API return. It passes with this change; with the generated guard reverted it aborts with Could not cast value of type 'NSNull' (0x...) to 'test_plugin.AllNullableTypes' (0x...).
• dart test and flutter_plugin_tools format pass locally.

Fixes flutter/flutter#191254

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request updates Pigeon to version 27.3.3. It modifies the Swift generator to check if a Flutter API response is nil or NSNull when validating non-nullable return values, preventing crashes when FlutterStandardReader substitutes NSNull for nil. It also updates the generated Swift files, adds a regression test in NullableReturnsTests.swift, and adds a generator unit test in swift_generator_test.dart. There are no review comments, so no further feedback is provided.