chore(docs): turn on strictNullChecks (#36180)

strictNullChecks was off for docs, which lets errors slip through and
leads to incorrect required/optional typing on Zod-inferred types. This
PR enables strictNullChecks and fixes all the existing violations.

apps/docs/components/Navigation/NavigationMenu/NavigationMenu.Context.tsx

@@ -3,7 +3,7 @@
 import { createContext, useContext } from 'react'
 
 interface ContextProps {
-  activeRefItem: string
+  activeRefItem: string | undefined
   setActiveRefItem: (x: string) => void
 }
 
@@ -15,7 +15,7 @@ interface Provider extends ContextProps {
 // createContext matches the shape that the consumers expect!
 const NavMenuContext = createContext<ContextProps>({
   activeRefItem: undefined,
-  setActiveRefItem: undefined,
+  setActiveRefItem: () => {},
 })
 
 export const NavigationMenuContextProvider = (props: Provider) => {