fix(bot): 🐛 Error on empty messages.

payphone/cogs/calls/cog.py

@@ -628,7 +628,12 @@ class Calls(base.Cog):
             message (discord.Message): The message received.
         """
 
-        if message.author.bot or message.is_system() or len(message.stickers) > 0:
+        if (
+            message.author.bot
+            or message.is_system()
+            or len(message.stickers) > 0
+            or not message.content
+        ):
             return
 
         if (await self.bot.get_context(message)).valid: