[BUG] Empty string id is not correctly validated by forceId

Description/Steps to reproduce

• define a model with forceId: true
• associate this model to a mongodb data source
• call the API to create a new model with an explicit empty string id

curl -X POST --header "Content-Type: application/json" --header "Accept: application/json" -d "{
  \"id\": \"\"
}" "http://localhost:3000/api/foos"

• this call creates a db entry with empty id and returns: {"id": ""}

Seems related to #1453 (see my comment)

Link to reproduction sandbox

https://github.com/simonbrunel/loopback-sandbox/tree/bug/empty-id-mongodb

Expected result

{
  "error": {
    "name": "ValidationError",
    "status": 422,
    "message": "The `foo` instance is not valid. Details : `id` can't be set (value: \"\").",
    "statusCode": 422,
    "details": {
      "context": "foo",
      "codes": {
        "id": [
          "absence"
        ]
      },
      "messages": {
        "id": [
          "can't be set"
        ]
      }
    }
  }
}

Additional information

node v6.11.3, Windows 10 64bit

npm ls --prod --depth 0 | grep loopback
loopback-sandbox@1.0.0
+-- loopback@2.39.0
+-- loopback-boot@2.27.0
+-- loopback-component-explorer@2.7.0
+-- loopback-datasource-juggler@2.55.3

Reproduce the issue using the linked loopback-sandbox branch with a model configured with forceId: true and a MongoDB data source. Trace the API create path and forceId validation for an explicit empty id; done means the request is rejected with the documented 422 ValidationError instead of creating a database entry.