[JavaScript] Model default value of `null` for strings results in `"null"` value

Description

Model default value of null for strings gets set to "null" upon creating a new instance of the model.

Swagger-codegen version

2.3.0

Swagger declaration file content or url

Schema definition for the model property:

outbound_localization : {
  "default": null,
  "description": "...",
  "type": "string"
}

Command line used for generation

docker run --rm -v ${PWD}:/local swaggerapi/swagger-codegen-cli generate \
      -i https://apidev.telnyx.com/mission_control_docs \
      -l javascript \
      -o /local/src/.swagger-codegen-output/missionControl \
      --additional-properties usePromises=true,useEs6=true

Steps to reproduce

1. Create a new version of the Connection model let connection = new moduleName.Connection();
2. See that the connection.outbound_localization value is "null"

Suggest a fix/enhancement

I suggest that if the default value for a model property is null, then set the value to null regardless of the property type. Otherwise keep it as undefined.

Use the JavaScript generation command and the generated Connection model as the entry points. Reproduce the issue with the schema property whose default is null, then trace how that default is rendered for a string. Done means a new model instance has null for that property rather than "null", while properties without a default remain undefined.