I have a simple Azure PowerShell runbook script workflow CheckIdentityColumns { Write-Output "Test Output" } 
When I am trying to test it I don't see any output. 
Why?
I tried your scenario and it worked for me. You can view the details below.
This happened to me in different scenario. The answer I got from Microsoft support was that the Runbook in the cache is still the older one, i.e. an empty Runbook in your case. All you need to do is: