ElasticSearch Ingest Pipeline: create and update timestamp field

Question

To create a timestamp field on my indices, according to this answer, I have created a Ingest Pipeline to run over specific indices:

PUT _ingest/pipeline/auto_now_add
{
  "description": "Assigns the current date if not yet present and if the index name is whitelisted",
  "processors": [
    {
      "script": {
        "source": """
          // skip if not whitelisted
          if (![ "my_index_1",
                 "my_index_2"
              ].contains(ctx['_index'])) { return; }
          
          // always update updated_at
          ctx['updated_at'] = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());
          
        """
      }
    }
  ]
}

then I apply to all indices settings as the default pipeline

PUT _all/_settings
{
  "index": {
    "default_pipeline": "auto_now_add"
  }
}

After that, I start indexing my objects into those indices. When I query an indexed item, I will get that item with the updated_at field updated at the time of the indexing like:

{
  _index: 'my_index_1',
  _type: '_doc',
  _id: 'r1285044056',
  _version: 11,
  _seq_no: 373,
  _primary_term: 2,
  found: true,
  _source: {
    updated_at: '2021-07-07 04:35:39',
    ...
  }
}

I would like now to have a created_at field, that only updates the first time, so I have tried to upsert script above in this way:

PUT _ingest/pipeline/auto_now_add
{
  "description": "Assigns the current date if not yet present and if the index name is whitelisted",
  "processors": [
    {
      "script": {
        "source": """
          // skip if not whitelisted
          if (![ "my_index_1",
                 "my_index_2",
                 "..."
              ].contains(ctx['_index'])) { return; }
          
           // always update updated_at
          ctx['updated_at'] = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());
          // don't overwrite if present
          if (ctx != null && ctx['created_at'] != null) { return; }
          
          ctx['created_at'] = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());
        """
      }
    }
  ]
}

but this solution does not seem to work: the condition

if (ctx != null && ctx['created_at'] != null) { return; }

will always fail, thus resulting in a update of the created_at at every object update on the index, in the same way of the updated_at field, making it useless. So, how to prevent that, and make sure that that field created_at exists after it has been created by the Ingestion Pipeline?