Followup to 7fc8d74f3eed52116add452b5321b41e02e04499
We need to check that the user export data exists before creating a model, since having an empty model will cause the UI to incorrectly think there's a user export to display.
This change shows the correct message, instead.
The GDPR requires all users to be able to export their data, or request an export of their data. This is fine for active users as we have a data export button on user profiles, but suspended users have no way of accessing the data export function, and the workaround for admins to export data for suspended users involves temporarily unsuspending them, then impersonating the user to export the data as them.
Since suspended users no longer have access to their account, we can safely assume that the export request will be coming via a medium outside of Discourse (eg, email). This change is built with this workflow in mind.
This change adds a new "User exports" section to the admin user page, allowing admins to start a new export, and to download the latest export file.
