fix(@remix-run/react): preserve null properties for useLoaderData and useActionData inference

.changeset/nervous-walls-sing.md

@@ -0,0 +1,10 @@
+---
+"remix": patch
+"@remix-run/react": patch
+---
+
+Fix inferred types for `useLoaderData` and `useActionData` to preserve `null`s.
+
+Previously, `null`s were being replaced by `never`s due to usage of `NonNullable` in `UndefinedOptionals`.
+Properties that aren't unions with `undefined` are now kept as-is, while properties that _do_ include `undefined`
+are still made optional, but _only_ remove `undefined` from the property type whereas `NonNullable` also removed `null`s.

packages/remix-react/components.tsx

@@ -1364,12 +1364,20 @@ type SerializeObject<T> = {
   >;
 };
 
-type UndefinedOptionals<T> = Merge<
+/*
+ * For an object T, if it has any properties that are a union with `undefined`,
+ * make those into optional properties instead.
+ * 
+ * Example: { a: string | undefined} --> { a?: string}
+ */
+type UndefinedOptionals<T extends object> = Merge<
   {
-    [k in keyof T as undefined extends T[k] ? never : k]: NonNullable<T[k]>;
+    // Property is not a union with `undefined`, keep as-is
+    [k in keyof T as undefined extends T[k] ? never : k]: T[k];
   },
   {
-    [k in keyof T as undefined extends T[k] ? k : never]?: NonNullable<T[k]>;
+    // Property _is_ a union with `defined`. Set as optional (via `?`) and remove `undefined` from the union
+    [k in keyof T as undefined extends T[k] ? k : never]?: Exclude<T[k], undefined>;
   }
 >;