chore(tests): Add json.Unmarshal test with empty value cases

[wawan93]

I've added some test cases in JSON for documentation purposes.
Only "00000000-0000-0000-0000-000000000000" and null values are parsed without errors. Empty strings (even with omitempty tag) are parsed with the error invalid UUID length.

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[noahdietz]

Empty strings (even with omitempty tag) are parsed with the error invalid UUID length.

I'm new to this project but @bormanp is this ideal? My gut says that deserializing a JSON blob that omits a UUID field into a struct that states omitempty for that same UUID field shouldn't produce an error and instead should just be Nil. If we fix this, we fix it in a separate PR of course. Even without omitempty I don't think it should be an error, just Nil.

Edit: I guess omitempty is for serialization not deserialization, so it's not a distinction for Unmarshal.