[Bender]

Backup script completes successfully but creates empty backup files

The cron job itself would need to do sanity checks on results. e.g. comparison of before / after directory sizes, file counts, perhaps a few canary files that never change and then alter the exit status based on all of those checks after performing some math logic as well as trigger monitoring alerts via your preferred mechanism. Your script can control the exit status. Some use functions that perform sanity checks, cleanup traps, etc... and with each failure add a number to '$?' assuming bash adding text output to the end of the script to describe the failures when calling the script in verbose mode.

In other words, whatever you the human did to realize there is a problem have the script perform the same checks as if it were you and alter the exit status and/or perform whatever other alerting methods are available to you.

If changing the exit status be sure the script is idempotent as some cron daemons may try to re-run the script depending on specific exit status. In other words if run a second consecutive time determine what you really want the script to do. Read up on the cron daemon you are using and how it interprets exit status and what it will do.