avoid TypeError on when registering service workers without config

Created by: peterbe

Fixes #5299 (closed)

I have no idea how to unit test this. Just trying to get the ball rolling.

packages/react-scripts/template/src/serviceWorker.js

@@ -68,7 +68,7 @@ function registerValidSW(swUrl, config) {
               console.log('New content is available; please refresh.');
 
               // Execute callback
-              if (config.onUpdate) {
+              if (config && config.onUpdate) {
                 config.onUpdate(registration);
               }
             } else {
@@ -78,7 +78,7 @@ function registerValidSW(swUrl, config) {
               console.log('Content is cached for offline use.');
 
               // Execute callback
-              if (config.onSuccess) {
+              if (config && config.onSuccess) {
                 config.onSuccess(registration);
               }
             }