Bug: missing `undefined` union in the withMigration account root & profile types

[gdorsi]

Both .root and .profile are currently typed as Type | null.
However, inside withMigration, the code does:

if (account.root === undefined) {
  // ...
}

That's because on the first migration the account.root can be undefined, as special exception over the other situatiuons.

We should provide a special type in withMigration to hint that both account.root and account.profile might be undefined.