Fix error when no valid messages can be found for selection

bot.py

@@ -123,6 +123,9 @@ async def random_message_command(ctx: commands.Context, channel: Optional[discor
         channel = ctx.channel
     messages = async_filter(_is_message_allowed_for_operations, channel.history(limit=max_cnt))
     messages = [item async for item in messages]
+    if not messages:
+        await ctx.channel.send("No valid messages found!")
+        return
     msg: discord.Message = random.choice(messages)
     author: discord.abc.User = msg.author
     embed = discord.Embed(