[go_router] Makes GoRouterState lookup more robust.

[chunhtai]

fixes https://github.com/flutter/flutter/issues/132824

Pre-launch 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 linked to 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.
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• [ ] I updated/added relevant documentation (doc comments with ///).
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• [ ] All existing and new tests are passing.

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