guard against result being undefined

Author: brian-mann

lib/extend/tag.js

@@ -184,7 +184,7 @@ NunjucksAsyncBlock.prototype.run = function(context, args, body, callback) {
   body((err, result) => {
     // wrapper for trimBody expecting
     // body to be a function
-    body = () => result
+    body = () => result || ''
 
     this._run(context, args, trimBody(body)).then(function(result) {
       callback(err, result);