Remove the default value of Webhook.
Before this repair, the default value of the `body` field of Webhook is `${payload}`,
but there is no `payload` field in the available fields of other events except message
publishing in the rule, so in this case, the webhook will send a string with the
message body as "undefined" to the HTTP service.
This fix removes the default value of the `body` field. When the `body` field is
not configured, Webhook will send all available fields of the current event in
the format of JSON object.