Fix for issue magento/magento2#18630

[dverkade]

Description (*)

When in the core_config_data table the setting for tax/defaults/postcode is NULL the compare on the frontend is breaking because NULL != " ". This fix resolves that issue.

Fixed Issues

1. Postcode / Zipcode in checkout form already validated on page load  #18630: Postcode / Zipcode in checkout form already validated on page load

Manual testing scenarios (*)

See steps in issue to reproduce original issue.

[magento-engcom-team]

Hi @dverkade

Unfortunately, we cannot accept Pull Requests from branches created in magento2 repository.
Please use your own fork as a base for this Pull Request.

Thank you.

[magento-engcom-team]

Hi @dverkade. Thank you for your contribution
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