VMware: fix compute node exception when no hosts in cluster

Commit 4033c0c9c16d2844b36fd5627717e7ce206887f6 casues the
regression. The result was a empty string and not None.
That is suds returns a Text object and not None.

Change-Id: I79e70e300f40eb3561cf56478d578f0f8cda273e
Closes-bug: #1383305
Merge "VMware: fix compute node exception when no hosts in cluster"