Protecting a Branch is ignored when no repository exists

Summary

When an a project is created without a Git Repository, you are allowed to setup a protected branch within the UI.

However, upon saving this by clicking the Protect button, the page refreshes and the protections never persist.

This is a problem when you want to protect branches prior to an initial commit - specifically when we're aiming to mirror an external repository but protect the branches initially.

Steps to reproduce

1. Create an empty project
2. Add a protected branch and save
3. Validate that the protection disappears and without any warning.

Example Project

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes