Unset network_section should be empty hash

Rather than a nil value, because the populated value is normally a hash. We
don't use the contents - the existing test was just documenting the current
behaviour.
More defensive coding for vApps without networks

Integration tests have been failing in a slightly different way on a vApp
that doesn't have the top-level keys `NetworkConfigSection` and
`ovf:NetworkSection`.

Change Vcloud::Walker::Resource::Vapp to be more defensive and explicit
about these behaviours. Extend the tests to document as such.
Merge pull request #86 from alphagov/68542054-more_empty_network_fixes

[Fixes #68542054] More defensive coding for vApps without networks