fix: Fix dereference of undefined (#363)

Fixed dereference of undefined on fnReturn when user function does not
return/returns undefined.

Fixed error in logging.

Signed-off-by: Matej Vašek <matejvasek@gmail.com>

lib/invoker.js

@@ -34,7 +34,7 @@ module.exports = function invoker(opts) {
 
         // If the response is a CloudEvent, we need to convert it
         // to a Message first and respond with the headers/body
-        if (fnReturn instanceof CloudEvent || fnReturn.constructor?.name === 'CloudEvent') {
+        if (fnReturn instanceof CloudEvent || fnReturn?.constructor?.name === 'CloudEvent') {
           try {
             const message = HTTP.binary(fnReturn);
             payload.headers = { ...payload.headers, ...message.headers };
@@ -121,7 +121,7 @@ module.exports = function invoker(opts) {
 };
 
 function handleError(err, log) {
-  log.error('Error processing user function', err);
+  log.error(`Error processing user function: "${err}"`);
   return {
     code: err.code ? err.code : 500,
     response: err.message