fix exec missing addpayload,

improve switch null test (to include null object)
check mqtt has payload before sending

nodes/core/core/75-exec.js

@@ -23,6 +23,7 @@ module.exports = function(RED) {
     function ExecNode(n) {
         RED.nodes.createNode(this,n);
         this.cmd = (n.command || "").trim();
+        if (n.addpay == undefined) { n.addpay = true; }
         this.addpay = n.addpay;
         this.append = (n.append || "").trim();
         this.useSpawn = n.useSpawn;