[CustomCom] [p]cc show - Handle missing cooldowns (#4112)

The old dataconverter didn't add cooldowns by default and `[p]cc show` expects them to be present

redbot/cogs/customcom/customcom.py

@@ -490,7 +490,7 @@ class CustomCommands(commands.Cog):
             command_name=command_name, author=author, created_at=cmd["created_at"], type=_type
         )
 
-        cooldowns = cmd["cooldowns"]
+        cooldowns = cmd.get("cooldowns", {})
 
         if cooldowns:
             cooldown_text = _("Cooldowns:\n")