Adjust error handling code

Author: yawaramin

src/htmx.js

@@ -2057,8 +2057,8 @@ var htmx = (function() {
   function handleTriggerHeader(xhr, header, elt) {
     const triggerBody = xhr.getResponseHeader(header)
     if (triggerBody.indexOf('{') === 0) {
-      const triggers = parseJSON(triggerBody)
-      for (const eventName of Object.keys(triggers || {})) {
+      const triggers = parseJSON(triggerBody) || {}
+      for (const eventName of Object.keys(triggers)) {
         let detail = triggers[eventName]
         if (isRawObject(detail)) {
           // @ts-ignore