resource-list fails on a failed resourcegroup stack

resource-list was failing if the stack fails when the ResourceGroup
resource is still in 'INIT_COMPLETE' status.
This change adds a check to make sure that nested stack is not None.

Change-Id: I7cbd5159fb544bda7c38b959c2b1b037bfaa4349
Closes-Bug: #1316705
Merge "resource-list fails on a failed resourcegroup stack"