BUG: Empty lists shouldn't be counted as DateOffsets. See #13844
- [x] closes #13844 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

tests!
 Ah, sorry. Could you point me in the direction of the relevant test file? The structure is different to numpy...
 [contributing docs](http://pandas.pydata.org/pandas-docs/stable/contributing.html)

this is prob in `tseries/tests_timeseries.py` though maybe in `tests/series/test_*` (somewhere) as well. you will have to find something similar and put right after.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13889?src=pr) is 85.31% (diff: 100%)

> Merging [#13889](https://codecov.io/gh/pydata/pandas/pull/13889?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13889   diff @@
==========================================
  Files           139        139          
  Lines         50138      50143     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42769      42777     +8   
+ Misses         7369       7366     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3186fef...cb26d25](https://codecov.io/gh/pydata/pandas/compare/3186fef545b55ca7b4a3c79c800f32b1d586545e...cb26d2555b231342eb7ce23fabef2b34e4bfb980?src=pr)
 Thanks. Right I've inserted a test in each of the files you mentioned. To be honest the logical distinction between the different classes in the test files wasn't that clear to me, so please say if they are in the wrong segment. 

First time contributing to pandas so just finding my feet!
 minor points. ping on green.
 thanks, will amend
 lgtm. pls add a whatsnew note in Bug Fixes.
 thanks!
