[w3c/payment-request] Sensible defaults for dictionary members (#780)

As part of https://github.com/heycam/webidl/issues/76, we should give dictionary members that accept other dictionary members a default value. In most cases, we set the default to `null` and IDL conversion treats that as if an empty dictionary was passed "{}". 

So, for example:

```JS
const dict = {};
response.retry(dict);
```

Would show up internally as if the following had been passed: `{ payer: {}, shippingAddress: {} }`. 


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