refs #7502. Small modification to a really bad code block

In the process of adding tests, thus far, I have managed to almost completely avoid making any modifications to the script files going under test. I could not avoid this one however, because otherwise I'd need to have workspaces in the ADS with very specific names, only to satisfy the clean-up routine. My simple fix is just to check that the workspaces exist before trying to delete them.
Merge remote-tracking branch 'origin/feature/7502_test_combine_multi'