[BUG][KOTLIN][TYPESCRIPT] Non nullable properties generate as nullable/undefined

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When we generate a client in either Kotlin or Typescript all properties from models become nullable (in Kotlin) or undefined (in Typescript). This means that some models may have nullable/undefined types when they are in fact always defined/not null.

openapi-generator version

CLI version 2.5.2.

OpenAPI declaration file content or url

For the sake of simplicity I've created the following spec file with only one model to be generated:

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenAPI definition",
    "version": "v0"
  },
  "servers": [
    {
      "url": "http://localhost:8080",
      "description": "Generated server url"
    }
  ],
  "paths": {},
  "components": {
    "schemas": {
      "AccountDto": {
        "type": "object",
        "properties": {
          "email": {
            "type": "string",
          }
        }
      }
    }
  }
}

Generation Details

I use the following CLI command to generate the code (for Kotlin):

npx @openapitools/openapi-generator-cli generate -I spec.json -c config.json  -g kotlin -o output/

With the following config file (for Kotlin):

{
  "packageName": "com.domain.project",
  "artifactId": "project",
  "groupId": "com.domain.project",
  "enumPropertyNaming": "UPPERCASE",
  "serializationLibrary": "jackson"
}

Steps to reproduce

1. Run the CLI command as described above with the provided config file.
2. Open the AccountDto model.
3. Check the generated property email.

You can see that the class will be generated like this:

data class AccountDto (
    @field:JsonProperty("email")
    val email: kotlin.String? = null
)

Instead of this:

data class AccountDto (
    @field:JsonProperty("email")
    val email: kotlin.String
)

Suggest a fix

Provide a way to make properties not nullable. I've tried to add these properties to the email property without success:

"email": {
  "type": "string",
  "nullable": false,
  "default": "email@email.com"
}

Start by running the provided CLI command with the sample spec and config, then compare the generated AccountDto output for Kotlin and TypeScript. Review how each generator interprets the schema's property and required information, and identify the existing generator tests covering nullable or undefined model properties. Done means the expected nullability behavior is reproduced and verified in tests for both targets.