docs: def _pyxb_from_perm_dict(self, perm_dict):
          """Return an AccessPolicy PyXB representation of ``perm_dict``
  
          - If ``norm_perm_list`` is empty, None is returned. The schema does not allow
          AccessPolicy to be empty, but in SystemMetadata, it can be left out
          altogether. So returning None instead of an empty AccessPolicy allows the
          result to be inserted directly into a SystemMetadata PyXB object.
  
          """
          norm_perm_list = self._norm_perm_list_from_perm_dict(perm_dict)
          return self._pyxb_from_norm_perm_list(norm_perm_list)