code: String getEffectiveNamespace() {
      String namespace = Tenant.getNamespace();
      if (namespace == null) {
        namespace = datastore.getOptions().getNamespace();
      }
      return namespace;
    }
docs: Returns the effective namespace. If a namespace was specified using {@link Tenant}, it will be
  returned. Otherwise, the namespace of this EntityManager is returned.
  
  @return the effective namespace.