check topic length > 0 before publish

to close #4414

packages/node_modules/@node-red/nodes/core/network/10-mqtt.js

@@ -104,6 +104,7 @@ module.exports = function(RED) {
      * @returns `true` if it is a valid topic
      */
     function isValidPublishTopic(topic) {
+        if (topic.length === 0) return false;
         return !/[\+#\b\f\n\r\t\v\0]/.test(topic);
     }