UI fix for empty panel for non-existent SLO id

[mgiota]

Summary

When creating a dashboard panel via Dev Tools with a Single SLO Overview embeddable pointing to a non-existent slo_id, the panel shows an empty loading spinner indefinitely instead of the "SLO has been deleted" message.

POST kbn:/api/dashboards/dashboard?apiVersion=1
{
      "title": "Single SLO dashboad with invalid slo_id",
      "panels": [
        {
          "config": {
              "slo_id": "1111",
              "overview_mode": "single"
           },
           "grid": {
              "x": 1,
              "y": 0
           },
           "type": "SLO_EMBEDDABLE"
          }
      ]
}

Root cause

In slo_overview.tsx, the "not found" check was unreachable dead code. The loading guard if (isRefetching || isLoading || !slo) always matched first because !slo is true when the SLO is undefined (not found). This returned the spinner before the isSloNotFound check could execute.

Fix

Reordered the conditions so the "not found" check (!isLoading && !isRefetching && slo === undefined) runs before the loading spinner guard. Now when loading completes and the SLO does not exist, the user sees the proper message: "The SLO has been deleted. You can safely delete the widget from the dashboard."

Before

After