BUG/API: Return a Series from DataFrame.asof with no match
 - [x] closes #15118
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Technically this is an API change, but more of a bug so didn't think there was a need to warn / deprecate?

Also there seems to be room to clean up the whole `asof` implementation, xref #10343, but wanted to keep that separate
minor comments, lgtm.

ok on the API change (usually these are more 'visible' even if its a 'bug fix') merge on green. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15232?src=pr) is 86.31% (diff: 100%)
> Merging [#15232](https://codecov.io/gh/pandas-dev/pandas/pull/15232?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15232   diff @@
==========================================
  Files           139        139          
  Lines         51091      51094     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          44099      44102     +3   
  Misses         6992       6992          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [2619ee3...5c1befe](https://codecov.io/gh/pandas-dev/pandas/compare/2619ee3cbf88b274d475b9a73d2d1e6e72c0a9ab...5c1befe93ca6d49ef5fb97c199f57a4cc4d791ee?src=pr)