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.utils.ts

@@ -1,7 +1,7 @@
 'use client'
 
-import { useEffect, useState } from 'react'
 import { usePathname } from 'next/navigation'
+import { useEffect, useState } from 'react'
 import { MenuId } from '~/components/Navigation/NavigationMenu/NavigationMenu'
 import type { ICommonItem } from '~/components/reference/Reference.types'
 import type { Json } from '~/features/helpers.types'
@@ -52,7 +52,10 @@ export function deepFilterSections<T extends ICommonItem>(
  *
  * See https://webpack.js.org/api/module-methods/#dynamic-expressions-in-import
  */
-export function useCommonSections(commonSectionsFile: string) {
+export function useCommonSections(
+  commonSectionsFile: string,
+  { enabled = true }: { enabled: boolean }
+) {
   const [commonSections, setCommonSections] = useState<ICommonItem[]>()
 
   useEffect(() => {
@@ -67,6 +70,10 @@ export function useCommonSections(commonSectionsFile: string) {
     fetchCommonSections()
   }, [commonSectionsFile])
 
+  if (!enabled) {
+    return null
+  }
+
   return commonSections
 }