Fix bindings in Storage

src/WebStorageAPI/Storage.res

@@ -5,14 +5,14 @@ Returns the name of the nth key, or null if n is greater than or equal to the nu
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Storage/key)
 */
 @send
-external key: (storage, int) => string = "key"
+external key: (storage, int) => Null.t<string> = "key"
 
 /**
 Returns the current value associated with the given key, or null if the given key does not exist.
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Storage/getItem)
 */
 @send
-external getItem: (storage, string) => string = "getItem"
+external getItem: (storage, string) => Null.t<string> = "getItem"
 
 /**
 Sets the value of the pair identified by key to value, creating a new key/value pair if none existed for key previously.

tests/WebStorageAPI/Storage__test.res

@@ -0,0 +1,14 @@
+open WebAPI.Global
+open WebAPI.Storage
+
+for i in 0 to localStorage.length - 1 {
+  localStorage->key(i)->Null.getOr("nothing")->Console.log
+}
+
+let item1 = localStorage->getItem("foo")->Null.getOr("nothing")
+
+localStorage->setItem(~key="bar", ~value="...")
+
+localStorage->removeItem("bar")
+
+localStorage->clear