euca-describe-properties: deal with missing property lists

euca-configure-vmware calls the back end code for
euca-describe-properties directly, but it doesn't supply a list of
properties to ask the server for.  The changes introduced in 21f9e28
(which were reverted in 731bc5f and un-reverted in 780a9aa) didn't
handle that well.

Fixes EUCA-6080