PartMap fails with converter=None

[EmmaTinten]

Using multipart and Partmap results in a NoneType error.

To Reproduce
Create a simple multipart def:

@multipart
@post('engine-rest/deployment/create')
 def deployment_create(
      self,
      **params: PartMap,
): 
      pass

and use it. Will produce an error:

site-packages/uplink/converters/keys.py", line 70, in <genexpr>
    return dict((k, converter(value[k])) for k in value)
TypeError: 'NoneType' object is not callable

Expected behavior
The submitted params should work just like the QueryMap.

[CodyTeague]

Seeing the same issue with FieldMap which is fixed by the above pull request.

    @returns(MergeRequestDetailSchema)
    @response_handler(raise_for_status)
    @put(
        "projects/{project_id}/merge_requests/{merge_request_iid}",
        args={
            "project_id": Path("project_id"),
            "merge_request_iid": Path("merge_request_iid"),
            "assignee_ids": Field("assignee_ids[]"),
            "merge_request_data": FieldMap,
        },
    )
    def update_merge_request(
        self,
        project_id: int,
        merge_request_iid: int,
        assignee_ids=None,
        **merge_request_data,
    ) -> MergeRequestDetail:

    merge_request = get_api().update_merge_request(
  File "/usr/local/lib/python3.9/site-packages/uplink/builder.py", line 95, in __call__
    self._request_definition.define_request(request_builder, args, kwargs)
  File "/usr/local/lib/python3.9/site-packages/uplink/commands.py", line 284, in define_request
    self._argument_handler.handle_call(
  File "/usr/local/lib/python3.9/site-packages/uplink/arguments.py", line 151, in handle_call
    self.handle_call_args(request_builder, call_args)
  File "/usr/local/lib/python3.9/site-packages/uplink/arguments.py", line 156, in handle_call_args
    annotation.modify_request(request_builder, call_args[name])
  File "/usr/local/lib/python3.9/site-packages/uplink/arguments.py", line 181, in modify_request
    converted_value = converter(value) if converter else value
  File "/usr/local/lib/python3.9/site-packages/uplink/converters/keys.py", line 71, in convert
    return dict((k, converter(value[k])) for k in value)
  File "/usr/local/lib/python3.9/site-packages/uplink/converters/keys.py", line 71, in <genexpr>
    return dict((k, converter(value[k])) for k in value)