Fix: Irrelevant 'beside' warning for bar plot without groups

[zeileis]

Fixes #370

Only throw the warning regarding beside=FALSE (rather than TRUE) in the presence of negative y values if there are by groups without facet_by being enabled.

[grantmcdermott]

Thanks @zeileis. If you don't mind, I'm going to close in favour of #372 (which I had started on my local computer before seeing your PR), since it simplifies the codebase in a few other places too. The specific solution for the beside case is the exact same, though.