Fix unused slack_webhook_url causing error

[npwolf]

I used slack_notification_enabled = false and was not supplying a slack_webhook_url. This resulted in the following error:

│ Error: encrypting with KMS Key (arn:aws:kms:us-xxxx-x:xxxx:key/xxxx-xxxx-xxxx-xxxx-xxxxxxx): InvalidParameter: 1 validation error(s) found.
│ - minimum field size of 1, EncryptInput.Plaintext.
│
│
│   with module.xxxx.module.xxxx.module.rds-pg.aws_kms_ciphertext.slack_url[0],
│   on .terraform/modules/xxxx.database.rds-pg/main.tf line 242, in resource "aws_kms_ciphertext" "slack_url":
│  242: resource "aws_kms_ciphertext" "slack_url" {
│