[BUG] [flask-python] Optional date in query parameters results in dateutil error when None

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
  4.3.0
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

I've recently extended my api with a query parameter of type string, format date. The parameter is optional. If the client does not fill in the parameter, it is None on the Python side, resulting in a parse error in dateutil:

Parser must be a string or character stream, not NoneType

openapi-generator version

4.3.0

OpenAPI declaration file content or url

paths:
  /events:
    get:
      tags:
        - events
      summary: "Get a list of all events"
      operationId: listEvents
      parameters:
        - $ref: '#/components/parameters/startDateParam'
components:
  parameters:
    startDateParam:
      name: startDate
      in: query
      description: Optional start date when querying date based items
      required: false
      schema:
        type: string
        format: date

Command line used for generation

openapi-generator generate -i ./api/openapi.yaml -g python-flask -o ./server/generated/ -c ./server/codegen/config.json

Steps to reproduce

• Generate server with above yaml and command line
• Start flask app
• Go to Swagger UI (/ui)
• Use events list call without filling in startDateParam
• See error as shown above

Related issues/PRs

Couldn't find a related issue.

Suggest a fix

It should either be fixed in util.mustache by adding a check for the date being not None and otherwise returning None. Or something similar in controller.mustache.

Same goes for datetime I guess, but haven't tested that.

Start by inspecting modules/openapi-generator/src/main/resources/python-flask/util.mustache and controller.mustache, then generate the sample Flask server with an optional date query parameter using the reported command. Reproduce the Swagger UI request without startDate and verify that the generated server accepts None without the reported dateutil error.