Importing a template without user data ruins the tl_files table

To reproduce:

1. Install Contao 3.1.2
2. Remove all *tl_user* and *tl_user_group* data from the file *templates/music_academy.sql*
3. Go to the install tool and import the template *music_academy.sql*
4. Create the admin account

After these steps the database table *tl_files* is wrong. All files and directories appear twice.