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. 
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15043?src=pr) is 84.75% (diff: 100%)
> Merging [#15043](https://codecov.io/gh/pandas-dev/pandas/pull/15043?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15043   diff @@
==========================================
  Files           145        145          
  Lines         51146      51146          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43350      43351     +1   
+ Misses         7796       7795     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6eb705f...9a70034](https://codecov.io/gh/pandas-dev/pandas/compare/6eb705f5350a0eeffd9e7663d7b4bf54acb4d6af...9a7003439bfe21b807b25493d7c38f98414cc184?src=pr) Tests moved and all green.  @Thanks!