ModuleApi: Should threepid properties `validate_at` and `added_at` be 0?

This issue has been migrated from [#7699](https://github.com/matrix-org/synapse/issues/7699).

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This was spun out of https://github.com/matrix-org/synapse/pull/7684

When `ModuleApi.register()` has `emails=` set to some emails, those emails are created and added to the new user's account.

However, in the database `validated_at` and `added_at` are set to 0, which some other may misinterpret.

Is this intentional or an oversight?

I've at least added the behaviour to a test here: https://github.com/matrix-org/synapse/pull/7684/files#diff-f7ba2d6448431a208d9b515b904ed854R49-R50 Which should be updated as part of closing this issue.