'synchronize' doesn't work with an empty repository

[amonks]

Synchronizing with an empty repository makes a 409 error

GET https://api.github.com/repos/[user]/[repo]/git/refs/heads/master?page=1&per_page=30 409 (Conflict)

to reproduce:

1. click "New Repository" in flowhub ui
2. don't add a readme or .gitignore
3. return to flowhub and click "synchronize"