Reset to default functionality for Checkout Branding Admin API mutation

As of Admin API 2024-01, you can use the checkoutBrandingUpsert mutation to reset branding settings to their default state without resetting each leaf field explicitly. It is now possible to pass in a null value to the checkoutBrandingInput argument to clear all of the branding settings, which will revert the branding of the Checkout to its default state.

Note that for all API versions, it is also possible to pass in a null value to a non-leaf subfield, for example design_system.colors.schemes.scheme1, to reset a given group of parameters.

Both of these changes enhance the usability of the Checkout Branding API by allowing to easily reset groups of branding parameters.