(stable) Prevent creation of empty repository identities
Summary: Fixes issue reported in https://secure.phabricator.com/rPf191a66490b194785fae28c062b71be99bb14584#43240
Test Plan: Imported an SVN repo, observed clean import instead of daemon exception.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D19466