Avoid nil pointer for 2.0 workflows

[mthx]

No description provided.

[mthx]

Looking at the amount of change in get.go since 2.0, I guess it's intentional that it doesn't support 2.0 workflows, but it's puzzling to fail with a nil pointer. Perhaps this PR is the best that can be done?

[gaganapplatix]

Yeah, 2.1 changed completely how the nodes are represented inside the workflow and how dependency is represented between nodes to allow generic support for dag and steps, which is why cli had to change significantly. I think we probably need to help detect this better and not fail with nil pointer. I will work with @jessesuen to figure out the best approach for 2.1