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