fix for - fog fails to load vdc with 1 or 0 vapps

Fog code fails to describe the vdc with zero or single vapps(resource
entities).
This is because the underlying API does not return Array, when vdc
has single or no vapp. Added a fix for this.
corrected the check for empty string

checking for an empty string , in case when vdc with zero resource entities are
returned
Merge pull request #2166 from snehaso/master

[vcloud-director] fixed fog to load vdc with single or zero resource entities