Python None in client list must be treated as SQL NULL
Python None in client list must be treated as SQL NULL

 - this ensures that when no list is passed in through the API (i.e.
   the `clients` key is not present in the passed-in JSON), the
   default value (i.e. an empty list) is used, rather than JSON `null`;
 - behaviour changed in SQLALchemy 1.1, see
   <http://docs.sqlalchemy.org/en/latest/changelog/migration_11.html#json-null-is-inserted-as-expected-with-orm-operations-omitted-when-not-present>
 - relates to [#135220913]

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api