docs: def _delete_null_columns(self):
          """
          executes a delete query to remove columns that have changed to null
          """
          ds = DeleteStatement(self.column_family_name)
          deleted_fields = False
          for _, v in self.instance._values.items():
              col = v.column
              if v.deleted:
                  ds.add_field(col.db_field_name)
                  deleted_fields = True
              elif isinstance(col, Map):
                  uc = MapDeleteClause(col.db_field_name, v.value, v.previous_value)
                  if uc.get_context_size() > 0:
                      ds.add_field(uc)
                      deleted_fields = True
  
          if deleted_fields:
              for name, col in self.model._primary_keys.items():
                  ds.add_where_clause(WhereClause(
                      col.db_field_name,
                      EqualsOperator(),
                      col.to_database(getattr(self.instance, name))
                  ))
              self._execute(ds)