BUG: fix DatetimeIndex._maybe_cast_slice_bound for empty index (GH14354)
Closes #14354

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14501?src=pr) is 85.26% (diff: 100%)

> Merging [#14501](https://codecov.io/gh/pandas-dev/pandas/pull/14501?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14501   diff @@
==========================================
  Files           140        140          
  Lines         50670      50670          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43205      43206     +1   
+ Misses         7465       7464     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e3d943d...304ca64](https://codecov.io/gh/pandas-dev/pandas/compare/e3d943d1876e8914036e9c323f1876e52da34a04...304ca6419d86c9e97ac290ef208fe9906ecf9a6e?src=pr)
 lgtm. (though travis is failing)
