[go_router_builder] Fixes an deprecated warning for using withNullability

[raju8000]

List which issues are fixed by this PR. You must list at least one issue.
Issue #166603

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• I [linked to at least one issue that this PR fixes] in the description above.
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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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