Add empty object defaults to stores that use sources

Avoids inconsistent state where the store might set `null` or
`undefined` due to the server’s response

js/stores/user_store.js

@@ -26,7 +26,7 @@ class UserStore {
         }
     }
 
-    onSuccessFetchCurrentUser({users: [user]}) {
+    onSuccessFetchCurrentUser({users: [user = {}]}) {
         this.userMeta.invalidateCache = false;
         this.userMeta.err = null;
         this.currentUser = user;

js/stores/webhook_store.js

@@ -28,7 +28,7 @@ class WebhookStore {
         this.getInstance().lookupWebhooks();
     }
 
-    onSuccessFetchWebhooks({ webhooks }) {
+    onSuccessFetchWebhooks({ webhooks = [] }) {
         this.webhookMeta.invalidateCache = false;
         this.webhookMeta.err = null;
         this.webhooks = webhooks;

js/stores/whitelabel_store.js

@@ -25,7 +25,7 @@ class WhitelabelStore {
         }
     }
 
-    onSuccessFetchWhitelabel({ whitelabel }) {
+    onSuccessFetchWhitelabel({ whitelabel = {} }) {
         this.whitelabelMeta.invalidateCache = false;
         this.whitelabelMeta.err = null;
         this.whitelabel = whitelabel;