Pre-process certain special keys to convert them from None values
        into empty containers, and to turn strings into arrays of strings.

def _preprocess(project_dict):
        """Pre-process certain special keys to convert them from None values
        into empty containers, and to turn strings into arrays of strings.
        """
        handlers = {
            ('archive',): _list_if_none,
            ('on-run-start',): _list_if_none_or_string,
            ('on-run-end',): _list_if_none_or_string,
        }

        for k in ('models', 'seeds'):
            handlers[(k,)] = _dict_if_none
            handlers[(k, 'vars')] = _dict_if_none
            handlers[(k, 'pre-hook')] = _list_if_none_or_string
            handlers[(k, 'post-hook')] = _list_if_none_or_string
        handlers[('seeds', 'column_types')] = _dict_if_none

        def converter(value, keypath):
            if keypath in handlers:
                handler = handlers[keypath]
                return handler(value)
            else:
                return value

        return deep_map(converter, project_dict)