Don't give Mercurial empty string as a remote name

Summary:
Fixes T9807. We currently run commands like this in some cases:

hg push -r master ''

From T9807, it seems that older Mercurial treated '' in the same way it would treat no argument, while newer Mercurial does not.

Passing '' is unusual and not intended.

Test Plan:
From T9807, @cspeckmim confirmed that running this command without the '' works, and @jgelgens tested the patch itself.

I didn't actually run this code myself, since I don't have Mercurial 3.6.1 installed and the fix seems straightfoward.

Reviewers: chad

Reviewed By: chad

Subscribers: cspeckmim

Maniphest Tasks: T9807

Differential Revision: https://secure.phabricator.com/D14531