BUG: fix SparseSeries reindex by using Series implementation
 - [x] closes #15447
 - [x] tests added / passed
 - [x] passes ``git diff master | flake8 --diff``
 - [x] whatsnew entry

I abunded a bit with the tests for ``.reindex()`` (for ``.loc`` there were already quite a bit).

This is completely independent from the approach in #15448 , which also made sense but on which I will come back in a different PR. In the current PR, I'm just exploiting the fact that sparse ``SparseBlock``s "know what to do", and the ``SparseSeries`` itself has nothing particular to do for reindexing.