Serdes v2: Handle missing User refs by synthesizing dummy users (#25391)

Many entities have `creator_id` and similar fields. `User`s are not
serialized. Foreign keys to users are serialized as email addresses.

During deserialization in a different instance (eg. a local dev instance
importing a dump from a prod instance) many such users may not exist.
This change creates new `User` entities on the fly with empty names,
generated passwords, and the email set.