Fixed Slack service throwing error when post is empty
Fixed Slack service throwing error when post is empty

fixes #11694

- if the post contained no body, the `.replace` would throw an error
- converted to an if-statement instead of doing `|| ''` because there
  would be a floating full-stop

bitjson/Ghost,ErisDS/Ghost,e10/Ghost,JohnONolan/Ghost,bitjson/Ghost,janvt/Ghost,TryGhost/Ghost,e10/Ghost,ErisDS/Ghost,JohnONolan/Ghost,ErisDS/Ghost,JohnONolan/Ghost,TryGhost/Ghost,TryGhost/Ghost,janvt/Ghost