Missing nucleic side chains displays incorrectly for ribbon / cartoon

[BradyAJohnston]

If style ribbon / cartoon is used on nucleic acids that don't have side chains, then the current styles still try to instance and rotate bases even though they shouldn't. This happens if the model is just missing side chains (4V6X as an example) or if Is Backbone is used as a selection for nucleic ribbon / cartoon.

We should detect that the bases aren't there and just not display them to begin with.