[Python] client None value set query parameters into literal 'None'

Description

With the generated Python API client, None value in the parameter with string type has been literally transformed to "None" and added to API call, because in Python string representation of None is "None"

But given that None in Python generally means a non-exist value, the expected behavior should not pass the value in query parameters or set the parameter value with a default value if specified.

Swagger-codegen version

2.4.17

Swagger declaration file content or url

If you post the code inline, please wrap it with

...
parameters:  [{
    "name": "test",
    "in": "query",
    "required": false,
    "type": "string"
  }]
...

-->

Command line used for generation

  java -jar swagger-codegen-cli-2.4.17.jar generate -l python -i swagger.json

Steps to reproduce

We generated the swagger.json file using swagger annotation (@ApiParam) from a Dropwizard application written in Java.

Suggest a fix/enhancement

Add None check in api.mustache.

{{#queryParams}}
        if '{{paramName}}' in params and params['{{paramName}}'] is not None:       
                query_params.append(('{{baseName}}', params['{{paramName}}'])){{#isListContainer}}  # noqa: E501
{{/queryParams}}

Start with modules/swagger-codegen/src/main/resources/python/api.mustache and inspect how optional query parameters are appended in generated Python clients. Generate a client from a definition containing an optional string query parameter, then verify that passing None omits the parameter rather than sending the literal value "None".