Ignore about scheme

[kiftio]

What changes are you making?

Prevent calling onCheckoutLinkClicked() for the about scheme, e.g. about:blank

How to test

n/a

Before you merge

[!IMPORTANT]

• [x] I've added tests to support my implementation
• [x] I have read and agree with the Contribution Guidelines.
• [x] I have read and agree with the Code of Conduct.
• [x] I've updated the README.

---

Checklist for releasing a new version

- [ ] I have bumped the version number in the [`build.gradle` file](https://github.com/Shopify/checkout-kit-android/blob/main/lib/build.gradle#L17) - [ ] I have added a [Changelog](https://github.com/shopify/checkout-sheet-kit-android/blob/main/CHANGELOG.md) entry. - [ ] I have updated the versions in the [README.md](https://github.com/shopify/checkout-sheet-kit-android/blob/main/README.md) for both Gradle and Maven.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.