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: Ib8e54ac9066b866133641fed10edba3209a38778
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>