Partially failing test for vApp without network

If a vApp fails during creation then it can be left as an empty shell
without any VMs or attached networks. The lack of networks causes
vcloud-walker to raise exceptions because certain parts of the vApp hash
aren't populated and we try to do operations on `nil` objects.

Mimic these with a unit test that modifies the Fog hash according to what
I've seen from the API. We probably only need to fix the handling of
`:NetworkConfig`. It should be OK to set `@network_section` to nil.

We originally thought that this might be caused by the lack of VMs but the
stacktrace in the story and further testing proves that it is only the
networks.
Fix handling of vApp empty :NetworkConfigSection

Default to an empty array if :NetworkConfigSection isn't available because
the vApp doesn't have any networks attached. This satifisies the input for
`#extract_network_config` which expects an array.
Merge pull request #80 from alphagov/63370428-fix_vapps_without_networks

[Fixes #63370428 #68542054] Fix handling of vApps without networks