fixes #7253 - change nil admin field on users to false, matches usergroups

When the admin field was nil, admin_changed? in user model validations can
evaluate to true if the field changed from nil to false.

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I have a test that I wrote for nil/false transitions prior to realising the schema was broken, but I think it's fairly pointless when the schema now prevents nil admins.  Let me know if you'd prefer it.

Marking as a 1.6.0 blocker as it involves a schema change and is a regression, I'd rather not put that in a point release.

Reproducer in the bug report comments.