sync.profile: Point dependencies to an empty string

This means that we are going to use the same branch name for the
dependencies as the branch of the repository under test.

If we are testing the 'stable' branch, all dependencies will point
to the 'stable' branch as well.

Change-Id: I8bef9c4860ad221a502411df669b4668c396a988
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>