fix: runtime error: invalid memory address or nil pointer dereference

[ArthurFritz]

Resolve error:
runtime error: invalid memory address or nil pointer dereference

The case of error occurs when in the documentation we have a reference to an object in the array null.

Exp Json (Generated of python Library auto documentation Swagger [drf-spectacular]):

"NullEnum": {
      "enum": [
          null
      ]
  }

Use this in property another object where of property type Enum is Nullable:

"profile_enum": {
                        "nullable": true,
                        "oneOf": [
                            {
                                "$ref": "#/components/schemas/ProfileEnumEnum"
                            },
                            {
                                "$ref": "#/components/schemas/NullEnum"
                            }
                        ]
                    }

I performed the YAML to JSON conversion test in the swagger-editor and was successful, with that I opened this suggestion so that it does not generate problems in the conversion.