[python-flask] Fix NoneType when deserialize optional date

Created by: LeComptoirDesPharmacies

What have been done

As "deserialize_date" and "deserialize_datetime" are used outside of "deserialize_model", we should check 'None' value before parsing string.

To close #5813

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Verify that it works

1. Generate a server : (Yaml file below) java -jar openapi-generator-cli/target/openapi-generator-cli.jar generate -i openapi.yaml -g python-flask -o ./server/generated/

2. Run : python3 -m openapi_server

3. Go to URL : Use Postman to "GET /events"

Without this fix :

[2021-08-02 17:41:35,566] ERROR in app: Exception on /events [GET]
[...]
TypeError: Parser must be a string or character stream, not NoneType

With this fix : 127.0.0.1 - - [02/Aug/2021 17:41:13] "GET /events HTTP/1.1" 200 -

openapi: "3.0.0"
info:
  version: 1.0.0
  title: OpenApi Petstore
  license:
    name: MIT
paths:
  /events:
    get:
      summary: "Get a list of all events"
      operationId: listEvents
      parameters:
        - $ref: '#/components/parameters/startDateParam'
      responses:
        '200':
          description: A paged array of pets
        
components:
  parameters:
    startDateParam:
      name: startDate
      in: query
      description: Optional start date when querying date based items
      required: false
      schema:
        type: string
        format: date

@taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @Jyhess (2019/01) @arun-nalla (2019/11) @spacether (2019/11)