Enum validation should not flag null values in nullable enums

[hkosova]

Q&A (please complete the following information)

• OS: Windows 10
• Browser: Chrome latest
• Method of installation: https://editor.swagger.io
• Swagger-Editor version: 3.8.0
• Swagger/OpenAPI version: OpenAPI 3.0

Content & configuration

Example Swagger/OpenAPI definition:

openapi: 3.0.2
info:
  version: 1.0.0
  title: Nullable enum
paths: {}
components:
  schemas:
    Myschema:
      type: object
      properties:
        nullableProp:
          type: string
          nullable: true
          enum:
            - null
            - 'value1'
            - 'value2'

Describe the bug you're encountering

This is a follow-up to PR #2079.

According to OpenAPI Specification maintainers (1, 2), nullable enums are defined as shown above. That is, the schema should have nullable: true AND also include null (not "null") in the list of enum values.

The issue is that the editor flags the null value and says "enum value should conform to its schema's type". But in this example, null does conform to the schema type, because nullable: true acts as a type modifier.

Expected behavior

null in the definition above should NOT trigger warnings.