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

packages/go_router/lib/src/delegate.dart

@@ -182,7 +182,7 @@ class GoRouterDelegate extends RouterDelegate<RouteMatchList>
 
   /// The top [GoRouterState], the state of the route that was
   /// last used in either [GoRouter.go] or [GoRouter.push].
-  GoRouterState? get state => currentConfiguration.last
+  GoRouterState get state => currentConfiguration.last
       .buildState(_configuration, currentConfiguration);
 
   /// For use by the Router architecture as part of the RouterDelegate.

packages/go_router/lib/src/router.dart

@@ -261,7 +261,7 @@ class GoRouter implements RouterConfig<RouteMatchList> {
   /// Accessing this property via GoRouter.of(context).state will not
   /// cause rebuild if the state has changed, consider using
   /// GoRouterState.of(context) instead.
-  GoRouterState? get state => routerDelegate.state;
+  GoRouterState get state => routerDelegate.state;
 
   /// Whether the imperative API affects browser URL bar.
   ///

packages/go_router/test/go_router_test.dart

@@ -6030,39 +6030,39 @@ void main() {
     await tester.pumpAndSettle();
 
     GoRouterState? state = router.state;
-    expect(state?.name, 'home');
-    expect(state?.fullPath, '/');
+    expect(state.name, 'home');
+    expect(state.fullPath, '/');
 
     router.go('/books');
     await tester.pumpAndSettle();
     state = router.state;
-    expect(state?.name, 'books');
-    expect(state?.fullPath, '/books');
+    expect(state.name, 'books');
+    expect(state.fullPath, '/books');
 
     router.push('/boats');
     await tester.pumpAndSettle();
     state = router.state;
-    expect(state?.name, 'boats');
-    expect(state?.fullPath, '/boats');
+    expect(state.name, 'boats');
+    expect(state.fullPath, '/boats');
 
     router.pop();
     await tester.pumpAndSettle();
     state = router.state;
-    expect(state?.name, 'books');
-    expect(state?.fullPath, '/books');
+    expect(state.name, 'books');
+    expect(state.fullPath, '/books');
 
     router.go('/tulips');
     await tester.pumpAndSettle();
     state = router.state;
-    expect(state?.name, 'tulips');
-    expect(state?.fullPath, '/tulips');
+    expect(state.name, 'tulips');
+    expect(state.fullPath, '/tulips');
 
     router.go('/books');
     router.push('/tulips');
     await tester.pumpAndSettle();
     state = router.state;
-    expect(state?.name, 'tulips');
-    expect(state?.fullPath, '/tulips');
+    expect(state.name, 'tulips');
+    expect(state.fullPath, '/tulips');
   });
 
   testWidgets('should allow route paths without leading /',