[go_router] Fix return type of current state getter to be non-nullable

[kaboc]

The state getter returns a GoRouterState?, but I don't think it needs to be nullable since currentConfiguration.last.buildState() never returns null.

I'm not sure if it is nullable on purpose for some changes planned in the Future. If there is a reason, it might be better to add a comment about it. Otherwise, it is just inconvenient that a null check is necessary.

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[kaboc]

Ah, is this a breaking change...? I'll revert the "Update version" commit if this fix should be included in the next major version rather than being released as a patch version.

[chunhtai]

Ah, is this a breaking change...? I'll revert the "Update version" commit if this fix should be included in the next major version rather than being released as a patch version.

no, I don't think so, at worse it will just generate a lint warning when the customer's code treats it as nullable

[chunhtai]

LGTM