def _convert_None_to_str(value):
    """
    returns input value or 'None' string value if input is None

    The DictParser representation of a pygeometa MCF can't handle an
    actual value of: None.  All dict keys must be strings.

    >>> _convert_None_to_str('foo')
    'foo'
    >>> _convert_None_to_str('None')
    'None'
    >>> _convert_None_to_str(None)
    'None'
    """
    if value is None:
        return 'None'
    return value