Split UPE - Prevent loading JS scripts when all payment methods are disabled

[james-allan]

Fixes #2865

Changes proposed in this Pull Request:

This PR fixes the following JS error if you disable all payment methods, including cards.

index.js:24 Uncaught TypeError: Cannot convert undefined or null to object
    at entries (<anonymous>)
    at ./client/blocks/upe/index.js (index.js:24:1)
    at __webpack_require__ (upe_blocks.js?ver=b65b5b1030ced18a6fe92b14ac363333:20:30)
    at upe_blocks.js?ver=b65b5b1030ced18a6fe92b14ac363333:84:18
    at upe_blocks.js?ver=b65b5b1030ced18a6fe92b14ac363333:87:10

Testing instructions

1. Go to WooCommerce → Settings → Payments → Stripe (Settings)
2. Enable UPE in the advanced section.
3. Go to the Payment methods tab and disable all payment methods (including cards).

5. Add a product to the cart and go to the block checkout page and view the browser console.
   • On add/deferred-intent branch you should see the error message above.
   • On this branch no error is observered.
6. Disable UPE in Stripe advanced settings, make sure that the the block checkout still works with and without Stripe enabled.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)

[wjrosa]

LGTM!

Before:

After: