fix: give dictionary members sensible defaults

[marcoscaceres]

*closes #780

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.
• [ ] Modified Web platform tests (link)
• [ ] Modified MDN Docs (link)

Implementation commitment:

• [ ] Safari (link to issue)
• [ ] Chrome (link to issue)
• [x] Firefox
• [ ] Edge (public signal)

Optional, Impact on Payment Handler spec? None.

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[marcoscaceres]

Closing this... IDL issue is still being discussed.