Fixed user-sent notifications

[applejag]

Changes proposed in this pull request:

• Fixed bug where user-sent notifications resulted in Python exception

I've no idea what values you all want in these params, so I'm just guessing. I made these changes inside the running container and now I can send notifications through the /#/vulnerability/details/{docker-image-name-and-tag} page by clicking the checkbox on a CVE and then the bell ("Notify")

Had to dig through the code a bit to understand, because the only thing that was logged out of the deepfence-api was:

[2022-08-03 14:08:32 +0000] [929] [ERROR] Error sending notification: 'notification_id'
ERROR:config.app:Error sending notification: 'notification_id'

@deepfence/engineering