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