[w3c/browser-payment-api] A PaymentDetailsModifier may not have an additionalDisplayItems value present (#335)

https://w3c.github.io/browser-payment-api/#constructor step 13 says:

> For each PaymentDetailsModifier in details.modifiers, if the additionalDisplayItems sequence contains any PaymentItem objects with an amount that is not a valid decimal monetary value, then throw a TypeError. 

But there may not be a value present for additionalDisplayItems at all in the dictionary.

Furthemore, the `amount` of a `PaymentItem` is not a string, so you can't check whether it's a "valid decimal monetary value".

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