Checkout Totals Sort Order fields can't be empty and should be a number

[barbanet]

If, for some reason, a backend user deletes the content of one or all Configuration -> Sales -> Sales -> Checkout Totals Sort Order fields, the checkout won't work and you won't see an error and no exceptions seems to be logged.

Description (*)

Now, field Subtotal, Discount, Shipping, Tax, Fixed Product Tax and Grand Total; inside Configuration -> Sales -> Sales -> Checkout Totals Sort Order, are required and can't be empty anymore.

Manual testing scenarios (*)

1. Go to Configuration -> Sales -> Sales -> Checkout Totals Sort Order
2. Remove all the values and save the configuration

3. Go the the storefront, add a product to the cart and try to access to the checkout. You'll see this page:

4. If you apply the validation from this PR you won't be able to save those fields with empty values.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[m2-assistant]

Hi @barbanet. Thank you for your contribution
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[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @miguelbalparda, thank you for the review.
ENGCOM-4798 has been created to process this Pull Request

[VasylShvorak]

✔️ QA passed

[ghost]

@Nazar65 unfortunately, only members of the maintainers team are allowed to assign developers to the pull request

[m2-assistant]

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