Enhancement: change `ModelResolver` to use `"nullable": true` for "non"-required properties based on default property inclusion of object mapper

As part of my SpringBoot application that uses the springdoc-openapi library (using swagger-core under the hood) to generate OpenAPI specs, I would like automatically generate .nullable(true) for "non"-required properties.

This because by (Jackson) ObjectMapper is configured with:

                .setSerializationInclusion(JsonInclude.Include.ALWAYS)

This causes Jackson includes null property values when it serializes JSON as opposed to omitted them.

But this means that every "non"-required property in de OpenAPI spec should also have "nullable": true.
However, the ModelResolver currently only adds a "nullable": true (based on what I digested from reading its source-code) in case nullable = true was specified to an @Schema(..) annotation.
But since I already use @NotNull annotations, I do not also want to also use @Schema(..) annotations.

What if we would change the ModelResolver to automatically adds nullable = true for "non"-required properties based on the "default property inclusion" in object mapper?
So that in case of JsonInclude.Include.ALWAYS the ModelResolver automatically adds nullable = true for "non"-required properties.

The "default property inclusion" could easily be determined using the (overloaded) getDefaultPropertyInclusion method of the SerializationConfig config (that can be retrieved calling .getSerializationConfig() on the object mapper.

References:

• https://github.com/springdoc/springdoc-openapi/issues/1275

Start by reading ModelResolver and the SerializationConfig methods available through the ObjectMapper, especially getSerializationConfig() and getDefaultPropertyInclusion(). Determine how non-required properties are resolved, then verify that schemas generated with JsonInclude.Include.ALWAYS mark those properties nullable without requiring @Schema annotations.