Payment error on NULL order balance, Order without any line items

I am leaning towards implementing PaymentAccessControlHandler::createAccess to check whether order balance is not null.

@zaporylie: +1 on the suggested approach. I do wonder whether we shouldn't also forbid adding a payment to a paid order?

I created a new access checker, as accessCreate doesn't have direct access to contextual route parameters.

@zaporylie: Thank you!

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