REF: initialize Series._name in _from_mgr to return valid Series object
Currently `Series._from_mgr` doesn't return a fully initialized Series object, because it misses the `_name` attribute (without this, several kinds of operations fail on the object). 
While `_from_mgr` is not a user-facing method, I think also from a developer perspective, it's nicer if it returns a proper Series, for example for during debugging, and for not having to think about setting `_name` after using it (as we are currently doing).
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Looks like this PR has gone stale and needs a rebase so closing to clear the queue, but feel free to reopen when you have time to circle back