Fix fatal when default_locale is empty - #142

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Changes proposed in this Pull Request:

Fixes a fatal error when trying to generate orders. Some countries may have an empty "default_locale" property. In which case, this would throw a fatal (at least in PHP 8).

There's actually, only one case of this here, so I guess I was "lucky" to hit this. This issue should be fixed separately in Woo itself, but checking here to avoid a fatal wouldn't be bad either IMO.

How to test the changes in this Pull Request:

1. Check out trunk
2. Run wp eval "var_dump( WC\SmoothGenerator\Generator\CustomerInfo::generate_person('MV') );"
3. See fatal
4. Check out this branch
5. Run wp eval "var_dump( WC\SmoothGenerator\Generator\CustomerInfo::generate_person('MV') );"
6. See success

Other information:

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• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Fix fatal when generating a large amount of orders, which increases the chances of hitting the empty locale issue.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.