email is not passed through to `check_registration_for_spam` hook with SSO registration

This issue has been migrated from [#10248](https://github.com/matrix-org/synapse/issues/10248).

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the `check_registration_for_spam` hook is passed a parameter `email_threepid`: "The email threepid used for registering, if any".

However, when a user signs up via SSO, there is no "threepid" per se, even if we get an email address from the SSO provider, so the spam checker does not see the email address.