fix(useRef typedef): allow null initialValue (#2281)

hooks/src/index.d.ts

@@ -50,7 +50,7 @@ type Ref<T> = { current: T };
  * Note that `useRef()` is useful for more than the `ref` attribute. It’s handy for keeping any mutable
  * value around similar to how you’d use instance fields in classes.
  */
-export function useRef<T>(initialValue: T): Ref<T>;
+export function useRef<T>(initialValue: T | null): Ref<T>;
 
 /**
  * `useRef` without an initial value is the special case handling `ref` props.