fix(hydrogen): call cart selector hook unconditionally in useOptionalCart

[malewis5]

WHY are these changes introduced?

useOptionalCart called the useCartSelector hook after an early
return undefined when rendered outside a <CartProvider>. That makes a
hook run conditionally, violating the Rules of Hooks: the number/order of
hooks changes between renders depending on whether a provider is present,
which can corrupt React's hook state and crash in dev.

WHAT is this pull request doing?

• useOptionalCart now always calls useCartSelector, passing a
  possibly-null store instead of returning early.
• useCartSelector accepts CartStore | null: getSnapshot returns
  undefined when there's no store, and it subscribes via a no-op
  subscribe so useRef/useSyncExternalStore always run in the same
  order.
• useCart continues to cast the result with as S, preserving the
  non-optional return type; useOptionalCart returns S | undefined.

No behavioral change for consumers inside a provider.

HOW to test your changes?

From packages/hydrogen:

pnpm test        # vitest — src/react/cart.test.tsx (26/26 passing)
pnpm typecheck