OAS 3.1.0 output: `maxLength` constraint not saved to schema for nullable string field

[bartvanandel]

Describe the bug We have several string fields in our API that are nullable (i.e., optional), but also have a max length. When using OAS 3.0.3 as the target schema version, this works fine, however with OAS 3.1.0, no maxLength is stored. For non-nullable fields, maxLength is stored properly.

I expect more constraints (e.g., minLength should be obvious) are also dropped from the schema. Format specifiers (such as type: email), readOnly, writeOnly and probably other specs work fine for nullable fields across OpenAPI versions, so there must be something special about constraint handling.

To Reproduce

Python code:

# Inside some model:
class Team(models.Model):
    phone_number = models.CharField(null=True, blank=True, max_length=40)

Excerpt from generated OAS 3.0.3 schema:

    TeamRequest:
      type: object
      properties:
        phone_number:
          type: string
          nullable: true
          maxLength: 40

Same excerpt from generated OAS 3.1.0 schema:

    TeamRequest:
      type: object
      properties:
        phone_number:
          type:
          - string
          - 'null'

Expected behavior Expected output should include maxLength constraint. If I understand the OpenAPI specs correctly (hints taken from this OpenAPI Spec question), this concise form should be fine:

    TeamRequest:
      type: object
      description: Team info
      properties:
        phone_number:
          type:
          - string
          - 'null'
          maxLength: 40

If you want to be more explicit, this should work as well:

    TeamRequest:
      type: object
      description: Team info
      properties:
        phone_number:
          oneOf:
          - type: string
            maxLength: 40
          - 'null'