Add mechanism to disable nullable for Kotlin properties

[0xabadea]

The newly-added KotlinNullablePropertyCustomizer results in Kotlin nullable properties being always marked as non-required and nullable. Sometimes that may not be a desirable output.

The Java/Kotlin+Jackson ecosystem does not generally or easily support undefined and null properties at the same time. Usually, Kotlin properties with a null value are either serialized as "property": null or they are left out (making them undefined). In our backend we leave them out via @JsonInclude non_null/non_absent and we document them as non-required in our OpenAPI spec. The understanding is that they may be missing from the payload, but if they are present, they are never null.

Our frontend then generates TypeScript types with optional properties, as in property?: TheType. In this setup, marking these properties as nullable in the OpenAPI spec results in optional and nullable TypeScript properties, as in property?: TheType | null. This brings no additional value and is actually detrimental, as the frontend developers now need to handle null values that never occur in practice.

I propose the introduction of a new property springdoc.model-converters.kotlin-nullable-property-customizer.enabled (defaulted to true) that would allow disabling KotlinNullablePropertyCustomizer on demand.

[fatso83]

This is highly anticipated 😉

[thejeff77]

For visibility on the wider plan: rather than landing a global flag for the customizer, I've proposed reverting #3256 (#3276) and adding a proper per-property override upstream in swagger-core.

• Issue: Proposal: introduce Schema.NullableMode enum to mirror RequiredMode / AccessMode pattern swagger-api/swagger-core#5160
• Implementation PR: feat: add Schema.NullableMode for explicit per-property nullable override (#5160) swagger-api/swagger-core#5161 — Schema.NullableMode { AUTO, NULLABLE, NOT_NULLABLE } mirroring the established RequiredMode / AccessMode pattern.

Rationale (full discussion on #3256): a global flag addresses the symptom, but the deeper bug is that users have no way to opt a single property out of auto-detection — and @Schema(nullable = false) cannot be honored because the boolean defaults to false and is indistinguishable from "not specified" via reflection. NullableMode fixes that the same way RequiredMode did for required in 2022. Acknowledging your work and pushback here directly led to this conclusion.

[0xabadea]

Hello @bnasslahsen, this PR has not been getting any attention so far.

Is there anything I can do to get merged either this or #3276 before the next version is released?