Fix nextItem not grabbing from correct storage.

source/item/item.test.ts

@@ -46,7 +46,7 @@ await setup(
 
       // Make sure `nextItem()` returns an item.
       let storedNext = await nextItem();
-      if (storedNext === undefined) {
+      if (storedNext === undefined || storedNext.value === undefined) {
         throw new Error("Expected an item");
       }
 

source/item/item.ts

@@ -178,6 +178,7 @@ export async function nextItem(): Promise<Value<Item> | undefined> {
     // means passing undefined here is fine as it won't be used.
     value: undefined!,
     serialize: serializeItem,
+    storage: storageForPrefix(itemKeyPrefix),
   });
 }