[REQ][kotlin-spring] Option to disable default value generation for optional model properties

Is your feature request related to a problem? Please describe.

The kotlin-spring generator adds = null to all optional properties in data classes:

  data class UserDto(
      val name: String,
      val email: String? = null,
      val role: RoleEnum? = null
  )

This causes two problems:

1. Silent data loss at scale. In projects with hundreds of models, default = null lets
   callers omit fields without any compiler warning. When a new field is added, every existing
   call site compiles fine and the missing field goes unnoticed until production.
2. Incorrect behavior with some libraries. Tools like MapStruct don't handle Kotlin default
   values correctly, leading to bugs in mapping logic.

Describe the solution you'd like

A config option (e.g. ommitDefaultNullValues) that omits = null defaults for optional
properties:

  data class UserDto(
      val name: String,
      val email: String?,
      val role: RoleEnum?
  )

Implementation:

https://github.com/OpenAPITools/openapi-generator/pull/23077

a switch in KotlinSpringServerCodegen.java + conditional wrap in
dataClassOptVar.mustache.

Describe alternatives you've considered

• Custom templates (--template-dir): works but requires maintaining a template fork across
  upgrades.

Start with KotlinSpringServerCodegen.java and dataClassOptVar.mustache, as identified in the issue, and review pull request 23077. Verify that the new configuration switch controls whether optional properties receive = null while preserving the existing default behavior.