[Kotlin] Models with "additionalProperties" result in invalid Kotlin code

Created by: jonpeterson

Description

When using the kotlin-spring generator, OpenAPI models including additionalProperties are generated as invalid Kotlin code.

openapi-generator version

5.4.0 and 6.0.1

OpenAPI declaration file content or url

components:
  schemas:
    MyObject:
      type: object
      properties:
        value:
          type: string
      additionalProperties: true
      required:
        - value

...is generated into...

data class MyObject(

    @field:JsonProperty("value", required = true) val `value`: kotlin.String,
) : kotlin.collections.HashMap<String, kotlin.Any>{

}

Note the missing () after kotlin.collections.HashMap<String, kotlin.Any>.

Suggest a fix/enhancement

I'd propose the fix as just adding the missing () to the kotlin-spring's dataClass.mustache file, but I'm not sure if it should be fixed there or in the general Kotlin code for the variable being provided to the template.