Use new validation option to return better label

Co-authored-by: Nick O'Leary <nick.oleary@gmail.com>

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

@@ -442,7 +442,17 @@
         }
         return defaultContentType || 'none'
     }
-
+    /**
+     * Test a topic string is valid for publishing
+     * @param {string} topic
+     * @returns `true` if it is a valid topic
+     */
+    function validateMQTTPublishTopic(topic, opts) {
+        if(!topic || topic == "" || !/[\+#\b\f\n\r\t\v\0]/.test(topic)) {
+            return true;
+        }
+        return RED._("node-red:mqtt.errors.invalid-topic");
+    }
     RED.nodes.registerType('mqtt-broker',{
         category: 'config',
         defaults: {