Steps to reproduce the issue

• Current nightly installation of J4.
• Activate error_log/error_reporting.
• Create an alternative module override file with a trailing number. E.g.:
  administrator/templates/atum/html/mod_custom/thing2.php
• Start Upload&Update using the J4 update package or full package. After entering the credentials and while the files are installed you see a Notice above the progress bar:
  PHP Notice: Undefined offset: -1 in administrator/components/com_templates/Model/TemplateModel.php on line 569
  or in your error_log file.