When selecting a notification on an event definition, it's possible to select no notification

When selecting a notification on an event definition, it's possible to select no notification and leave the box empty, resulting in a message "Could not find information for Notification" when creating the event, or an error when editing the event.

## Expected Behavior
Not selecting a notification should highlight the box and pop red error text asking the user to select a notification.

## Current Behavior
When creating an event:
- Allows you to not select a notification, resulting in a "Could not find information for Notification" entry
- Video: https://link.cekkent.net/98d6a26c-7e8c-4122-80e0-b5c6fa8fa17b

When editing an event:
- Throws an error: _Loading component failed: Cannot read properties of undefined (reading 'localeCompare')_
- Video: https://link.cekkent.net/87e4ae3c-8f92-46f9-840d-8cf4412c59bb

## Steps to Reproduce (for bugs)

When creating an event:
- Video: https://link.cekkent.net/98d6a26c-7e8c-4122-80e0-b5c6fa8fa17b
- Fill out Event Definition page, work to the Notification section
- Click Add Notification
- Make no selection for the notification, just click Done
- Notification apparently adds, but states "Could not find information for notification"

When editing an event:
- Video: https://link.cekkent.net/87e4ae3c-8f92-46f9-840d-8cf4412c59bb
- Edit the event
- Click Notification section
- Click Add Notification
- Make no selection for the notification, just click Done
- Event definition edit page disappears, only shows message: _Loading component failed: Cannot read properties of undefined (reading 'localeCompare')_

## Context

This contributes to a negative user experience by either creating an apparent successful yet ultimately inoperable notification (during event creation) or a mishandled error that apparently clears the screen (during event edit).

## Your Environment

* Graylog Version: Graylog 5.0.2+59d96f8
* Java Version: bundled
* Elasticsearch Version: Opensearch 2.4.1
* MongoDB Version: 5.0.14
* Operating System: Debian 11
* Browser version: Chrome Version 109.0.5414.87 (Official Build) (arm64)