Allow Optional type annotation

[ScarletBlizzard]

Error when calling Consumer method with return type annotation Optional[...] if response is None:

...
 File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/builder.py", line 100, in __call__
    self._request_definition.define_request(request_builder, args, kwargs)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/commands.py", line 287, in define_request
    self._method_handler.handle_builder(request_builder)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/decorators.py", line 62, in handle_builder
    annotation.modify_request(request_builder)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/returns.py", line 66, in modify_request
    converter = self._get_converter(request_builder, return_type)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/returns.py", line 57, in _get_converter
    return request_builder.get_converter(
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/helpers.py", line 96, in get_converter
    return self._converter_registry[converter_key](*args, **kwargs)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/converters/__init__.py", line 52, in __call__
    converter = self._converter_factory(*args, **kwargs)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/converters/__init__.py", line 112, in chain
    converter = func(factory)(*args, **kwargs)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/converters/typing_.py", line 127, in create_response_body_converter
    return self._base_converter(type_)
  File "/home/max/.cache/pypoetry/virtualenvs/autom-tnujRd_K-py3.10/lib/python3.10/site-packages/uplink/converters/typing_.py", line 121, in _base_converter
    if issubclass(type_.__origin__, self.typing.Sequence):
  File "/home/max/.pyenv/versions/3.10.13/lib/python3.10/typing.py", line 1158, in __subclasscheck__
    return issubclass(cls, self.__origin__)
  File "/home/max/.pyenv/versions/3.10.13/lib/python3.10/abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class

To Reproduce
Create a Consumer with method that consumes API endpoint that returns None (it is shown as "null" if using curl). Add Optional[SomeType] return type annotation to this method. Call this method to get the error

Expected behavior
Absence of this error. Instead, simply return None