BUG: don't raise on empty usecols
Title is self-explanatory.

yep this fine. ping on green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13402?src=pr) is **84.23%**

> Merging [#13402](https://codecov.io/gh/pydata/pandas/pull/13402?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13402   diff @@
==========================================
  Files           138        138          
  Lines         50805      50805          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42795      42796     +1   
+ Misses         8010       8009     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d405bf2...8eed8d1](https://codecov.io/gh/pydata/pandas/compare/d405bf26a03b3ecdaa30b62c4b1dd5cb22930329...8eed8d13bfc93883bd5b6e259e460693b419d1b4)
 @jreback: Travis is giving the green light.  Ready to merge if there are no concerns.
