Fixes #3263 - strip null-key entries from component schema properties…

[Shaveenblu]

When swagger-core resolves a @JsonUnwrapped bean property (e.g. Spring HATEOAS EntityModel), it uses JSON-based cloning internally. Schema.getName() is @JsonIgnore, so the name is lost during cloning. These null-named schemas are then inserted as null keys into the properties map, causing Jackson to throw JsonMappingException: Null key for a Map not allowed in JSON when serializing the OpenAPI document.

Add SpringDocUtils.removeNullKeySchemas(OpenAPI) which recursively removes any null-key entry from every component schema properties map, and call it in AbstractOpenApiResource.getOpenApi() after schema resolution and before the user-facing customizers run.

Five unit tests added in SpringDocUtilsTest covering top-level null keys, nested null keys, null OpenAPI, null components, and schemas with no properties.

[Mattias-Sehlstedt]

Hi, have we investigated the cause for this issue? My understanding from the github-issue is that it is entirely new with 3.0.3, and thus that it is a regression stemming from somewhere. If that is the case, I would find it suitable to first look for the cause of the regression, since it could affect many other things, before we introduce a "cleaner" that simply attempts to cover the issue up with additional post-processing.

It might for example be tied to this new way of conducting a copy. But it is of course easiest to test verify if we use Springdoc 3.0.2 and then bump only swagger-core (to tell if the change merely comes from the bump of swagger-core or if the issue resides in springdoc itself).