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: Id206bb81aa4bd719ac8818b98103238deb05435d
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>