code: public List<Descriptor<RepositoryBrowser<?>>> getBrowserDescriptors() {
          if(repositoryBrowser==null)     return Collections.emptyList();
          return RepositoryBrowsers.filter(repositoryBrowser);
      }
docs: Returns the list of {@link RepositoryBrowser} {@link Descriptor}
  that can be used with this SCM.
  
  @return
  can be empty but never null.