Also resolves T62668 [since that report was about not being able to
select the remaining curve -- which we dont draw now anymore ;)]
Compared to 2.79, this also has the benifit of actually showing the curve
again if you set both bevel start and end to 0.0 for example.
(note that we could also go the other way and decide to always draw the
curve, that would then mean having a look to include the curve in the
selection buffer I guess to resolve T62668)