Getting "Couldn\'t find notification for ID 000000000000000000000001" when trying to create a notification output with a custom value

## Expected Behavior
When we request api/events/notifications with a payload like this:

```json
{
"id":"000000000000000000000001",
"title":"GL Notification Opsgenie - Level P1",
"description":"This resource send P1 alerts to Opsgenie",
"config":{
"type":"legacy-alarm-callback-notification-v1",
"callback_type":"com.opsgenie.plugin.graylog.OpsGenieAlarmCallback",
"configuration":{
"api_key":"my_secret_api_key",
"tags":"foo, bar",
"teams":"my_awesome_team",
"api_url":"https://api.eu.opsgenie.com/v2/alerts",
"show_fields":"Application,gl2_message_id",
"priority":"P1"
}
}
}
```
We expected a notification output to be created with the ID 000000000000000000000001

## Current Behavior
Instead, We are receiving this error in the response:

```json
"Couldn\'t find notification for ID 000000000000000000000001"
```

## Context
We created an alerts pipeline where event definitions point to well know notifications IDs, in this way
it does not matter if we redeploy the graylog server and setup all notifications again, cause they keep the same notification ID

## Your Environment

* Graylog Version: 5.03
* Elasticsearch Version: 7.10.2
* MongoDB Version: 6.04
* Operating System: Ubuntu