TST: Test empty method and simplify logic
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

Since the docstring of `.empty` states

>  True if NDFrame is entirely empty [no items], meaning any of the axes are of length 0.

I was wondering if the logic could be changed accordingly for code clarity. Unless it was set as `not all(len(self._get_axis(a)) > 0 for a in self._AXIS_ORDERS)` for a particular reason.

Also this method did not appear to have any test. 