Unable to clear the hook url from the notification settings

[edmundito]

Actual:
When trying to remove the URL from the notification settings and saving, the user gets an error.

Expected:
The user should be able to clear the notification settings when removing the URL.

Steps to reproduce:

1. Go to notification settings
2. Add URL and save
3. Remove URL and save

[krishnaglick]

Likely just need to remove the frontend validation if no value is provided.