[BUG] "default: null" is considered to be an error even with "nullable: true"

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

OpenApiGenerator CLI considers default: null is not correct for an attribute even if it has nullable: true

openapi-generator version

OpenApiGenerator CLI 7.0.1

OpenAPI declaration file content or url

openapi: 3.0.3
info:
 title: test
 version: v0.1
 description: test
paths:
 /api/path:
  get:
   responses:
    '200':
      description: ok
      content:
       application/json:
        schema:
         $ref: '#/components/schemas/MyEntity'
components:
 schemas:
  MyEntity:
   type: object
   properties:
    someField:
     type: array
     items: 
       type: string
     nullable: true
     default: null

Generation Details

Steps to reproduce

java -jar openapi-generator-cli-7.0.1.jar generate --generator-name csharp --input-spec my_spec.yaml --output out_dir

(nb: I'm writing --generator-name csharp because I have to pick a generator name in order to have a valid command, but I'm observing the issue with every generator)

• Expected behavior: the command runs successfully (ie: it generates code and exits with 0 status code)
• Actual behavior: the command exits with 1 status code and display the error

 | Error count: 1, Warning count: 0                                           
Errors:
        -attribute components.schemas.MyEntity.default is not of type `array` 

Note that I would expect this command to work because:

• the tool https://apidevtools.org/swagger-parser/online/ considers my spec is ok
• according to the OAS spec :

default – the default value must conform to the specified schema.

and that value (null) conforms the schema because it has nullable: true

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/issues/16633 is related to this issue but is not really the same
• other issues found with search is:issue default null seems completely unrelated

Suggest a fix

Start by running the provided openapi-generator-cli 7.0.1 command with the YAML declaration and reproduce the validation error for components.schemas.MyEntity.default. Trace the CLI validation path to compare default: null with nullable: true; done means the declaration is accepted and generation exits with status 0 for the reported case.