Fixes error when a repos has zero commits.
Fixes error when a repos has zero commits.

scheduler caught exception:
undefined method `each' for "":String
lib/github_backend.rb:31:in `block in contributor_stats_by_author'
lib/github_backend.rb:23:in `each'
...

chillu/github-dashing,kostasdizas/github-dashing,kalabox/kalabox-dashboard,kalabox/kalabox-dashboard,CGATOxford/dashboard,CGATOxford/dashboard,kostasdizas/github-dashing,chillu/github-dashing,chillu/github-dashing,kostasdizas/github-dashing,kalabox/kalabox-dashboard,CGATOxford/dashboard