Empty schema description if polymorphic subtype missing Javadoc

[westse]

Describe the bug

• springdoc-openapi-javadoc causes an empty schema description to be generated for classes that are polymorphic subtypes and missing a Javadoc comment.

To Reproduce
Steps to reproduce the behavior: See linked pull request (#2371)

Expected behavior

• Schema description should remain null if class Javadoc is missing.

Additional context
Probably has the same cause as #2035 .