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: I6b6d5efa72d00be636668c393418990a5da45a1b
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>