[Fixes #68542054] 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.

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PS: I changed my mind and broke the tests out to separate examples.