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

Created by: Tohnmeister

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.