Fix showing the project overview if head is undefined, assume `master` then
    
    If `head` is undefined, the ajax request listing the branches available
    for PRs will fail because the `branch_to` argument will be `None` and
    thus ignored while it's actually required. So instead of bailing out
    just assume the head is `master`, the rest of the code should know how
    to deal with this.
    
    Fixes https://pagure.io/pagure/issue/3305
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>