`email_provider` by default is NULL, and if a user had not logged the
value would remain NULL. Upgrading to GitLab 10.0 would lead to a
PG::UniqueViolation because the post-deploy migration would attempt
to reinsert the entry because the NULL comparison between
`users.email_provider` and `user_synced_attributes_metadata.email_provider`
would never match.

Closes #38246