The only rights that can be stored on survey are Read writes.
Having a _convertWorkspaceToRootRightContexts that does not return null

• has one advantage : searching read rights on any context will be search on survey (meaning the user has no right elsewhere) : not interesting
• has one disadvantage : we are looking for all rights also on Survey all the time
• 
  So _convertWorkspaceToRootRightContexts should return null