Allow calling the cart .get() method right after cart mutation methods

[blittle]

WHY are these changes introduced?

Allow calling .get() immediately after cart mutation methods:

import {
  createCartHandler,
  cartGetIdDefault,
  cartSetIdDefault,
} from '@shopify/hydrogen';

const cartHandler = createCartHandler({
  storefront,
  getCartId: cartGetIdDefault(request.headers),
  setCartId: cartSetIdDefault(),
  cartQueryFragment: CART_QUERY_FRAGMENT,
  cartMutateFragment: CART_MUTATE_FRAGMENT,
});

await cartHandler.addLines([{merchandiseId: '...'}]);
// This change fixes a bug where `cart` would be null, even though a
// new cart was created when adding a line item
const cart = await cartHandler.get();

WHAT is this pull request doing?

When calling cart.get(), we only make a request if a cartId exists. The problem is, someone might call a mutation method that creates the cart, and then want to get the cart. If we know the cart has been created, we can internally store the cartId in memory, and only call getCartId() if that internal value is undefined.

HOW to test your changes?

Add await cart.get() right after await cart.addLines(inputs.lines) in cart.ts in the skeleton template..

Make sure the returned value is defined after adding an item to the cart. Also make sure that no cart exists before adding to the cart (clear cookies).

Post-merge steps

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation