list-volume fails in multi-vcenter environment if all nodes are in only one vcenter

[Mattes83]

/kind bug

What happened:
list-volume fails in multi-vcenter environment if there are only nodes in one vcenter.
The call to Retrieve should not be made if there are no properties to retrieve.

What you expected to happen:
The call to Retrieve should be skipped.

How to reproduce it (as minimally and precisely as possible):
Add two VirtualCenter entries to csi-vsphere.conf, but only have nodes in one VirtualCenter.

Anything else we need to know?:
I stumbled on this when migrating to a multi-vcenter setup.

Environment:

• csi-vsphere version: 3.0.1

[gohilankit]

@Mattes83 Can you paste the error logs you were getting?