fix: handle discriminator subschemas that only inherit via allOf without own properties

[amalv]

Description

This PR addresses an issue where the OpenAPI spec defines a discriminator with a subschema that only inherits all fields from the parent (using allOf) and does not define any new properties. Previously, this pattern would cause a runtime error in the Docusaurus OpenAPI plugin due to an attempt to access properties of an undefined object.

See #1132 for more background.

Motivation and Context

OpenAPI allows for discriminator mappings where a child schema inherits all properties from its parent via allOf and does not introduce any new properties. This is a valid and common pattern for modeling inheritance. Prior to this fix, such schemas would cause the documentation build or runtime to fail with a TypeError. This PR adds a safeguard to ensure the code handles these cases gracefully, allowing the documentation to render correctly for all valid OpenAPI discriminator patterns.

How Has This Been Tested?

• Added a minimal test case in demo/examples/tests/discriminator.yaml under /discriminator-empty-subschema that uses a parent schema with a discriminator and a child schema that only inherits via allOf.
• Verified that the documentation renders without errors and the schema is displayed as expected.

[github-actions]

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