[Fleet] `getBulkAssets` echoes whatever it receives

[consulthys]

Kibana version: 8.15.0

Elasticsearch version: 8.15.0

Server OS version: ESS

Browser version: Version 126.0.6478.185 (official build) (x86_64)

Browser OS version: MacOS 11.2.3

Original install method (e.g. download page, yum, from source, etc.): ESS

Describe the bug: The "Ingest pipelines" link available in Stack Monitoring stopped working as designed in 8.15.0.

If the elasticsearch integration assets have not been previously installed, clicking on that link redirects the user to a 404 page (Screenshot 1) instead of showing a popup inviting the user to install the assets (Screenshot 2)

Up to 8.14.3, getBulkAssets would not return an asset if it didn't exist as a saved object. Since 8.15.0, it now simply echoes back links for whatever asset is sent to it, whether fictitious or not . Since the response from that call is used as a condition to show the popup (screenshot 2) and the call always returns a non-empty response, regardless of the existence of the underlying asset, the popup is never shown.

Steps to reproduce:

1. Spin up a 8.15.0 stack (8.9.0 through 8.14.3 work fine)
2. Enable Stack Monitoring (on-prem) / Enable Logs & Metrics (in ESS)
3. Go to the Stack Monitoring application
4. Click on Overview
5. Click on the "Ingest Pipelines" link
6. Witness that you're redirected to the Dashboards application with a 404 message (screenshot 1)

Expected behavior: The expected behavior is to see a popup (screenshot 2) inviting the user to install the elasticsearch integration assets if those haven't been installed prior.

Screenshots (if relevant):

Screenshot 1: Dashboard 404

Screenshot 2: Popup inviting the user to install integration assets

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

On any version below 8.15.0, the following request returns an empty response:

POST kbn:/api/fleet/epm/bulk_assets
{
  "assetIds": [
    {
      "id": "elasticsearch-metrics-ingest-pipelines",
      "type": "dashboard"
    }
  ]
}

=> 
[]

As of 8.15.0, the same request returns whatever is passed:

POST kbn:/api/fleet/epm/bulk_assets
{
  "assetIds": [
    {
      "id": "elasticsearch-metrics-ingest-pipelines",
      "type": "dashboard"
    }
  ]
}
=>
{
  "items": [
    {
      "id": "elasticsearch-metrics-ingest-pipelines",
      "type": "dashboard",
      "attributes": {},
      "appLink": "/app/dashboards#/view/elasticsearch-metrics-ingest-pipelines"
    }
  ]
}

POST kbn:/api/fleet/epm/bulk_assets
{
  "assetIds": [
    {
      "id": "foo-bar",
      "type": "dashboard"
    }
  ]
}
=>
{
  "items": [
    {
      "id": "foo-bar",
      "type": "dashboard",
      "attributes": {},
      "appLink": "/app/dashboards#/view/foo-bar"
    }
  ]
}

Additional context:

Possibly related to https://github.com/elastic/kibana/pull/182180 as it seems to be the only one between 8.9.0 and 8.15.0 that modified the getBulkAssets function in a meaningful way.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)