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: I71e26192238cfd548a4f766d1fe469e390eea2a5
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>