Automatic merge from submit-queue
Fix detect-node-names to not error out if there are no nodes
Fixes#21564.
Teardown was not working correctly in rare cases because `detect-node-names` was failing before any of the actual cleanup was run. I'm pretty sure the issue was that there was an instance group, but no instances in the instance group, so we bailed out when we tried to expand the bash array.
This PR adds a guard so we don't bail if the array is empty.
cc @jlowdermilk @spxtr
f8196d9048