[go_router_builder]: Fixes issue when using query parameters that caused a nullable error

[Karibash]

The impact of #8405 caused a build error when defining a route with a required value.

Fixed: flutter/flutter#164358

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[Karibash]

I get an error when I try to sign the CLA, what should I do?

[Karibash]

Closing this as it is a duplicate of #8737.