insights: handle search alerts of an empty repository gracefully

Created by: coury-clark

When a search returns no repositories in a result it reports an alert:

{
  "data": {
    "search": {
      "results": {
        "alert": {
          "title": "No repositories found"
        },
        "results": []
      }
    }
  }
}

Currently the Code Insights background processor treats these alerts as transient errors, and triggers retry logic that ends up spamming a lot of queries that are guaranteed to return the same result. We should handle this alert more gracefully and avoid triggering the retry condition.