Just to be clear, the patch produces the following response in an empty
repo (which is not ideal but slightly better than before):
$ darcs push
No default repository to push to, please specify one.
$ darcs push ""
darcs failed: Not a repository: (/_darcs/inventory does not exist)
HINT: Do you have the right URI for the repository?
I don't care too much about the corner case `darcs push ""`, and the
patch improves the code of D.UI.Commands.Push, so I'm going to accept it.