Fix long-standing bug in aws.stringSetToPointers
Fix long-standing bug in aws.stringSetToPointers

Instead of N pointers, we were returning N null pointers, followed by the real
thing. It's not clear why we didn't trip on this until now, maybe there is a
new server-side check for empty subnetID strings.

kubernetes/cloud-provider-aws,kubernetes/cloud-provider-aws,kubernetes/cloud-provider-aws