Send Test Notification via DM fails

### Environment SaaS (https://sentry.io/) ### Steps to Reproduce 1. Create a new Issue Alert 2. Set action to Slack notification 3. Add Slack username & user ID 4. Click `Send Test Notification` ### Expected Result Receive Dummy Event via DM ### Actual Result <img width="935" alt="image" src="https://github.com/getsentry/sentry/assets/86369415/4bae5d82-a9c0-42d3-a62b-140c1528d4cb"> After saving the alert, the notification correctly sends an event via DM. So it seems to work but "Send test notification" is broken. ### Product Area Alerts ### Link _No response_ ### DSN _No response_ ### Version _No response_