Change `usePathname` to return `string | null` (#42380)
This changes the API of `usePathname` to return `string | null` to
support hybrid use-cases where the pathname is unknown at build time
(during automatic static optimization and when fallback is set true with
dynamic parameters in the pathname).
This supports a cleaner DX experience for those moving from `pages/` to
`app/` so they can begin to use `usePathname` in components that are
shared across them.
