Shorten Shop local storage store code

src/utils/stores/shop.ts

@@ -18,11 +18,9 @@ export const cartId: Writable<string> = writable(null)
 
 // Write to localStorage when changing cartId
 if (typeof localStorage !== 'undefined') {
-    const cartId = localStorage.getItem('cartId')
-    if (cartId) {
-        cartId.set(cartId)
-        // console.log('existing cart:', cartId)
-    }
+    const id = localStorage.getItem('cartId')
+    id && cartId.set(id ? id : null)
+
     cartId.subscribe(value => localStorage.setItem('cartId', value))
 }