flex could handle nil primitives better #64

The current behavior is to add a box that is visible. Probably it would be better to either add an invisible box, or to change the properties of that box so that it has the same background as the flex does. The overall default background is just not a great choice if someone has gone to the effort to style the flex itself.

It's easily worked around by following the documented advice, but I can't think of any good reason that the current default behavior is actually the most desirable behavior.

Happy to submit a PR for this... would be trivial to just make the box transparent, and only slightly harder to make it opaque but honor the flex background if set. Let me know.