Default postfix to empty string. https://github.com/Koenkk/zigbee2mqtt/issues/593

lib/extension/devicePublish.js

@@ -53,7 +53,7 @@ class DevicePublish {
         topic = topic.replace(`/${type}`, '');
 
         // Check if we have to deal with a postfix.
-        let postfix = null;
+        let postfix = '';
         if (postfixes.find((p) => topic.endsWith(p))) {
             postfix = topic.substr(topic.lastIndexOf('/') + 1, topic.length);