[w3c/payment-request] fix: set default for optional dictionaries (#874)

The following tasks have been completed:

 * [x] Confirmed there are no ReSpec errors/warnings.
 * [X] Modified Web platform tests - IDL changes will be picked up automatically.
 
Implementation commitment:

 * [ ] Safari (link to issue)
 * [ ] Chrome (link to issue)
 * [X] [Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1562680)
 * [ ] Edge (public signal)

Optional, impact on Payment Handler spec?
none.

@rsolomakhin, @danyao, the underlying IDL change is tracked at:  https://bugs.chromium.org/p/chromium/issues/detail?id=948139

@aestes, this change is a result from a change to WebIDL that introduces a default dictionary value of `{}` for when dictionaries are optional: https://github.com/heycam/webidl/pull/750

I'm not sure who to ping on the WebKit side about updating the binding layer. 

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-request/pull/874

-- Commit Summary --

  * fix: set default for optional dictionaries

-- File Changes --

    M index.html (8)

-- Patch Links --

https://github.com/w3c/payment-request/pull/874.patch
https://github.com/w3c/payment-request/pull/874.diff

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