js.html.Storage.getItem should take into account possible null values

## Description

The return type of the method `js.html.Storage.getItem()` is `String` in the [Haxe API documentation](https://api.haxe.org/js/html/Storage.html#getItem), but the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/API/Storage/getItem) state that it can also return `null`.

## Steps to reproduce the issue

Run [this snippet](https://try.haxe.org/#a1CbD24a) to see that the `Storage.getItem()` method can return `null`, so that the `@:nullSafety` macro can warn against null accesses.

```haxe
import js.Browser;

class Test {
static function main() {
final toggleDarkMode = Browser.window.localStorage.getItem("toggle-dark-mode");

trace(toggleDarkMode == null);
}
}
```

```
16:25:48:960 Test.hx:7:,true
```

## Expected behavior

The signature of `js.html.Storage.getItem()` is taking into account null values:

```haxe
getItem(key:String):Null
```