getBranch : Use master as default value

Introduction

The "published" variant has no branch information.

Current situation

In org.onehippo.repository.documentworkflow.task.GetBranchTask#doExecute the variantBranchId of published branch is always null, so it will never return the published variant.

Desired situation

In case of the "published" state the getBranch workflow tasks return the published variant if the branchId is "master".