fixing Nonetype not iterable error

[kapilkd13]

fixing this error https://ci.commonwl.org/job/cwltool-pr-conformance-multiver/543/version=v1.0/console . Error occures when we iterate over a empty list of args.testargs.

[mr-c]

Can you add a test for this fix?

[kapilkd13]

The code which caused this problem is inside main method and I can't seem to find a good way to test it, as once we invoke main method we would need a tool and workflow to successful execution. Also this particular issue was caused because we were reading a empty list, so i don't think a test is necessary for this. But since this issue arrived while all checks passed, so a test to run a particular workflow is needed.
I am thinking we can fetch a particular conformance test, install a previously released tagged version of cwltool(not current version, so that its behaviour does not change with time) and test that conformance test. Is this a good idea. If yes, I would work on it after windows compatibility is complete. For now I think we can merge this PR as current version is broken and will affect all cwltool builds. What do you say

[mr-c]

@kapilkd13 Sure! See #24