throws when variables_names_map is empty

[aaraney]

@seyounger discovered that an empty variables_names_map causes ngen to throw the following:

C++ exception with description "variables_names_map is a String, not an object and cannot be referenced as one." thrown in the test body.

This PR is just a test to showcase the failure.

I would expect that this would be a no-op.

[robertbartel]

My $0.02 here ...

Absent a problem I'm not seeing with the workaround, I don't think this behavior should be modified (or at least considered) in isolation (and so these tests should not be added currently). I'm pretty sure at least model_params behaves the same way. I do strongly suspect other elements are indeed no-ops if empty, though.

That's also not necessarily to say all these things should be consistent ... that seems reasonable, but it too should be more deeply examined. And honestly, I suspect the backend for configuration management could use some work.