[pigeon]fix a crash when casting NSNull to an Array

[hellohuanlin]

Fixes a crash introduced in #3658:

static func fromList(_ list: [Any?]) -> NativeAuthSession? {
  if let userPoolTokensList = list[2] as! [Any?]? {}
}

where list[2] is NSNull, which is not an Optional, hence can't be casted to [Any?]?.

Recall that nilOrValue helper is created for this purpose. So the fix is simply:

static func fromList(_ list: [Any?]) -> NativeAuthSession? {
  if let userPoolTokensList: [Any?] = nilOrValue(list[2]) {} // <- HERE
}

Why didn't we catch this regression

Missing unit tests - we did not test NSNull fields to ensure nilOrValue works for them.

Why did it work in previous version?

It's surprising that this worked fine before! The original code is:

static func fromList(_ list: [Any]) -> NativeAuthSession? {
  if let userPoolTokensList = list[2] as! [Any]? {}
}

From my previous PR, we know that list[2] is an implicit optional (that contains a NSNull value). I think Swift made an exception here when casting implicit optional NSNull (either intentionally or unintentionally).

List which issues are fixed by this PR. You must list at least one issue.

Fixes flutter/flutter#129283

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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[tarrinneal]

There's a unit tests that needs updated to match the new code output.

[tarrinneal]

I think we're going to need to add collections that contain nulls to the integration tests with this pr as well.

[stuartmorgan-g]

I think we're going to need to add collections that contain nulls to the integration tests with this pr as well.

Yes, we should definitely add an integration test that covers this. Unit tests are helpful, but can mislead us for Pigeon where we're dealing with cross-language marshalling (e.g., we've had cases where we unit tested both sides of a change, but there was a subtle difference in the protocol so it didn't actually work in practice).

[hellohuanlin]

@tarrinneal can you point me to a similar test as an example? (sorry im not yet familiar with this project)

[tarrinneal]

@tarrinneal can you point me to a similar test as an example? (sorry im not yet familiar with this project)

packages/pigeon/platform_tests/shared_test_plugin_code/lib/integration_tests.dart Is where all of our integration tests are run from.

packages/pigeon/pigeons/core_tests.dart is the pigeon definition file that you would update if there isn't already a method that you can use for these tests (which is likely)

and then each platform has it's own implementation code inside the test_plugin folder or alternate_language_test_plugin folders

[tarrinneal]

If you need more specifics I can help with that. If you don't have the bandwidth for this atm I can help as well.

[hellohuanlin]

If you don't have the bandwidth for this atm I can help as well.

@tarrinneal that'd be great. i'm indeed fully swamped this week. thanks a lot!

[tarrinneal]

@stuartmorgan think this is enough coverage on the integration tests?

[stuartmorgan-g]

LGTM with some nits.

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