getElementById could return null

Author: Freddy03h

src/DOMAPI/Document.res

@@ -15,7 +15,7 @@ Returns the first element within node's descendants whose ID is elementId.
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/getElementById)
 */
 @send
-external getElementById: (document, string) => element = "getElementById"
+external getElementById: (document, string) => null<element> = "getElementById"
 
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/getAnimations)

src/DOMAPI/DocumentFragment.res

@@ -38,7 +38,7 @@ Returns the first element within node's descendants whose ID is elementId.
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/DocumentFragment/getElementById)
 */
   @send
-  external getElementById: (T.t, string) => element = "getElementById"
+  external getElementById: (T.t, string) => null<element> = "getElementById"
 
   /**
 Inserts nodes before the first child of node, while replacing strings in nodes with equivalent Text nodes.