Fixed WOM offset when n=0
Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #20517
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20549?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@336fba7`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20549/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20549?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20549   +/-   ##
=========================================
  Coverage          ?   91.82%           
=========================================
  Files             ?      153           
  Lines             ?    49305           
  Branches          ?        0           
=========================================
  Hits              ?    45274           
  Misses            ?     4031           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <ø> (?)` | |
| #single | `41.89% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20549?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/20549/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20549?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20549?src=pr&el=footer). Last update [336fba7...581bfe6](https://codecov.io/gh/pandas-dev/pandas/pull/20549?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hi @jreback, I see travis fails https://travis-ci.org/pandas-dev/pandas/jobs/360472961, any idea whats going wrong?
 ping @jreback  Sorry @jreback, I'm wondering if can I do someting to get this task done? there is a lint issue @jreback the JOB="3.5_OSX" travis fails, it doesn't seem to be related with my changes...  thanks @mmngreco 