PWA-3172: [bug]: Regions select stuck in loading state in checkout when US is disabled

[Tarun50745]

Description

When United States is not selected in the backend General > Country Options > Allow Countries configuration, the checkout fails to load the "States" field in the checkout. This is because it passes US to the GetRegions by default but this returns a graphql error when US is not active. When there is only one country available this causes the checkout to fully break as there's no way to trigger the load of the states.

Related Issue

Closes https://jira.corp.adobe.com/browse/PWA-3172.

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Checklist

• I have added tests to cover my changes, if necessary.
• I have added translations for new strings, if necessary.
• I have updated the documentation accordingly, if necessary.

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:book:	Associated JIRA tickets: [PWA-3172](https://jira.corp.magento.com/browse/PWA-3172).
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