[BUG][validate] Component with a property pointing to a missing $ref incorrectly passes validation

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

Spec files with that have a component with a property pointing to a missing $ref are not currently flagged as an issue by the built-in validator.

openapi-generator version

The current latest docker image is digest 4e5bf573bce9.

OpenAPI declaration file content or url

Full file

I took this sample file and modified it like so:

18:18 $ git diff
diff --git i/samples/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml w/samples/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml
index 31100fda527..d828e997ec6 100644
--- i/samples/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml
+++ w/samples/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml
@@ -775,7 +775,7 @@ components:
           format: int64
           type: integer
         category:
-          $ref: '#/components/schemas/Category'
+          $ref: '#/components/schemas/DoesNotExist'
         name:
           example: doggie
           type: string
✔ ~/code/openapi-generator/samples [master|✚ 1]

Generation Details

N/A, this only deals with validation.

Steps to reproduce

In general:

1. Validate file with a component with a property pointing to a missing $ref
2. File passes validation when it shouldn't.

Specifically
First ensure a valid file:

1. Run docker run --rm -v $PWD/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml:/tmp/openapi.yaml openapitools/openapi-generator-cli:latest validate -i /tmp/openapi.yaml passes validation, as expected.
2. Run docker run --rm -v $PWD/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml:/tmp/openapi.yml stoplight/spectral lint /tmp/openapi.yml to confirm
3. edit $PWD/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml with above modifications, introducing a component with a property pointing to a missing $ref
4. Run docker run --rm -v $PWD/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml:/tmp/openapi.yaml openapitools/openapi-generator-cli:latest validate -i /tmp/openapi.yaml still passes validation, which is unexpected.
5. Run docker run --rm -v $PWD/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml:/tmp/openapi.yml stoplight/spectral lint /tmp/openapi.yml again and the error is correctly reported.

These edits also fail validation against https://apidevtools.org/swagger-parser/online

Full output of these commands is in this gist.

Related issues/PRs

I queried "ref validate" and found issues close to this, but none focused just on validation.

Suggest a fix

Components with a property pointing to a missing $ref should be considered invalid.

Start with the validate -i CLI command and reproduce the issue using samples/openapi3/server/petstore/python-flask/openapi_server/openapi/openapi.yaml, changing the Category reference to DoesNotExist. Compare the built-in validator result with Spectral and the swagger-parser result; done means the missing component property reference is reported as invalid.