Fix getAttribute return type

Author: nojaf

src/DOMAPI/Element.res

@@ -115,7 +115,7 @@ Returns element's first attribute whose qualified name is qualifiedName, and nul
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Element/getAttribute)
 */
   @send
-  external getAttribute: (T.t, string) => string = "getAttribute"
+  external getAttribute: (T.t, string) => null<string> = "getAttribute"
 
   /**
 Returns the qualified names of all element's attributes. Can contain duplicates.

tests/DOMAPI/Element__test.js

@@ -0,0 +1,6 @@
+external myElement: DOMAPI.element = "myElement"
+
+switch (myElement->Element.getAttribute("foo")) {
+| Null.Value(value) => value->Console.log
+| Null => "nothing"->Console.log
+}