LibWeb: Support [Reflect] on IDL String attributes that may return null

This change allows IDL interfaces to be compiled using new AK String
which have a attribute in the interface that may return null.

Without this change we would run into a compile error from code such as
the following example:

```
auto retval = impl->deprecated_attribute(HTML::AttributeNames::ref);

if (!retval.has_value()) {
    return JS::js_null();
 }
 return JS::PrimitiveString::create(vm, retval.release_value());
```

As `deprecated_attribute` returns a `DeprecatedString` instead of an
`Optional<String>`. Fix that by using the non-deprecated attribute
implementation, and falling back to the empty string for where we cannot
return null.

Also add a test here to cover a regression I almost introduced here
which was not previously covered by our test suite.

Ideally, all of this should actually just be calling
Element::get_attribute_value, but I'm not entirely sure at this stage
what the behavioral change would be to test for here. Since this
implementation preserves the previous behavior, stick with it, and add a
FIXME for now.

Tests/LibWeb/Text/input/idl-handling-of-null-attribute.html

@@ -0,0 +1,9 @@
+<script src="include.js"></script>
+<script id="script-element">
+    test(() => {
+        let s = document.getElementById("script-element");
+        println(`initial type = '${s.type}'`);
+        s.type = null;
+        println(`after setting to null = '${s.type}'`);
+    });
+</script>