String options in config parser convert null values to strings

Passing None to a string option should cause an error, but it returns a string instead:

>>> from teklia_toolbox import ConfigParser
>>> parser = ConfigParser()
>>> parser.add_option('dont_be_null', type=str)
>>> parser.parse_data({'dont_be_null': None})
{'dont_be_null': 'None'}

Using int, float, list, or dict results in a TypeError, but str(None) returns 'None'. Since we use the ConfigParser to parse YAML files, where None is written null, this means we translate null to 'None', which makes things even weirder.