Bug: Ref::FindDefaultBranchName returns `""` instead of `nil` for an empty repo

~Conversation: #220 (closed)

Reported by @yhl3692319 on the conversation

Hi @andrewn, when I created a new project, the project show got 500 error. The problem was that when to find default branch name via gitaly, golang return "" not nil, so it went error here:

= link_to namespace_project_new_blob_path(@project.namespace, @project, @project.default_branch || 'master') do