Regression: Generated Api Doc does not mark fields as required in Kotlin

[simonhauck]

Describe the bug

After Upgrading our Kotlin project to SpringBoot 4 and updating this library to 3.0.1 we noticed a change in the generated open-api specification.

Fields that are by Kotlin language definition Not-Null are now no longer marked as required. This worked in previous versions.

To Reproduce
Steps to reproduce the behavior:

• What version of spring-boot you are using: 4.0.1
• What modules and versions of springdoc-openapi are you using: 3.0.1
• What is the actual and the expected result using OpenAPI Description (yml or json)?

Actual openApi.json

{
  "openapi": "3.1.0",
  # ....
  "components": {
    "schemas": {
      "HelloWorld": {
        "type": "object",
        "properties": {
          "message": {
            "type": "string"
          }
        }
      }
    }
  }
}

Expected:

{
  "openapi": "3.1.0",
  # ...
  "components": {
    "schemas": {
      "HelloWorld": {
        "type": "object",
        "properties": {
          "message": {
            "type": "string"
          }
        },
        "required": [
          "message"
        ]
      }
    }
  }
}

• Provide with a sample code (HelloController) or Test that reproduces the problem

@RestController
@SpringBootApplication
class DemoOpenApiApplication{

	@GetMapping("/hello")
	fun hello(): HelloWorld {
		return HelloWorld("Helo world")
	}
}

data class HelloWorld(
	@field:NotNull val message: String)

fun main(args: Array<String>) {
	runApplication<DemoOpenApiApplication>(*args)
}

Expected behavior

Fields that are by Kotlin Language definition not null, should be marked as required

Additional context
A minimal reproducable example can be found here.