Allow explicitly null state in EndSessionRequest

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The OpenID Connect RP-Initiated Logout specification defines the state parameter as OPTIONAL. However, the current Android implementation of `flutter_appauth` makes it effectively mandatory due to the underlying behavior of the plugin's bridge logic with the AppAuth-Android and iOS.

# The Problem:
When calling `endSession` with an IdP that does not support/return the state parameter, the logout flow fails with a exception:
```dart
PlatformException(end_session_failed, Failed to end session: [error: null, description: Response state param did not match request state] ...)
```

## Technical Root Cause:
1. In the AppAuth-Android SDK, the EndSessionRequest.Builder constructor automatically generates a random, secure state string by default.
2. In the `flutter_appauth` Android plugin, the `state` from Dart is only passed to the native builder if it is NOT null.
3. Consequently, passing state `null` from Dart fails to clear the pre-generated state in the native builder. The request is sent with a state, the server returns without one, and AppAuth-Android throws an exception.

# Proposed Change:
Align the Android implementation with the OIDC specification by allowing the state to be explicitly set to null on the native code.

The `flutter_appauth` logic should be updated to ensure that if `null` (or something equivalent, maybe a empty string '' or having an additional boolean flag) is passed from Dart, the native builder.setState(null) is called to override the default generated state.

**NOTE**: The AppAuth-Android SDK explicitly added support for a nullable `state` (see Issue [#615](https://github.com/openid/AppAuth-Android/issues/615)) to support IdP providers that do not echo the _state_ back in the logout redirect. This change would expose that native capability to Flutter developers.

Start in the Android plugin bridge that maps Dart's endSession state to the AppAuth-Android EndSessionRequest.Builder. Trace how a null state is currently handled and verify the native builder receives an explicit nullable state. Done means a null Dart state can complete logout when the provider omits state, without triggering the state-mismatch exception.

Written by the indexing model from the issue text.