greeting: send an empty string as message content for falsy values (such as null) instead

src/greeting.js

@@ -13,7 +13,7 @@ function init(bot) {
     function sendGreeting(file) {
       bot.getDMChannel(member.id).then(channel => {
         if (! channel) return;
-        channel.createMessage(config.greetingMessage, file);
+        channel.createMessage(config.greetingMessage || '', file);
       });
     }