Unless it’s initial backup, it will more likely error out because all the expected files are missing.
If you want to ensure this can nver happen, you can use chattr to make your folder immutable.
When I tried an empty option in the GUI, it took that as a request to check a folder called “true”
in addition to the primary destination. The right folder given twice would also seem reasonable.
On Windows, I tested a non-existent drive letter and that worked too, so maybe lots of options.
Improving Reliability with Large Backup Sets gave a script you can use with run-script-before,
which should let you get quiet non-runs when the drive is missing (still waiting for test though).