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: I9b031859972b0996987eb4cc697b06b0fb569d89
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>