utility.banner: guild error handling

src/modules/utility.js

@@ -175,6 +175,9 @@ banner.callback = async function (msg, line) {
       return "`--server/--guild` can only be used within guilds.";
     } else {
       const guild = msg.channel.guild || hf.bot.guilds.get(msg.guildID);
+
+      if (!guild.banner) return "This guild does not have a banner.";
+
       const url = `${BANNER_BASE}${guild.id}/${guild.banner}.${
         guild.banner.startsWith("a_") ? "gif?size=1024&_=.gif" : "png?size=1024"
       }`;