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: I527d529456fba47fcb4e48c1129612d620878f0a
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>