branch: consider an empty upstream remote config as not found
Fixed active_refspecs field not initialized on new git_remote objects

When creating a new remote, contrary to loading one from disk,
active_refspecs was not populated. This means that if using the new
remote to push, git_push_update_tips() will be a no-op since it
checks the refspecs passed during the push against the base ones
i.e. active_refspecs. And therefore the local refs won't be created
or updated after the push operation.