getSelection could return null

Author: Freddy03h

src/DOMAPI/Document.res

@@ -442,7 +442,7 @@ Returns an object representing the current selection of the document that is loa
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/getSelection)
 */
 @send
-external getSelection: document => selection = "getSelection"
+external getSelection: document => null<selection> = "getSelection"
 
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/hasStorageAccess)

src/DOMAPI/Window.res

@@ -514,4 +514,4 @@ external cancelIdleCallback: (window, int) => unit = "cancelIdleCallback"
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Window/getSelection)
 */
 @send
-external getSelection: window => selection = "getSelection"
+external getSelection: window => null<selection> = "getSelection"

src/Global.res

@@ -744,4 +744,4 @@ external cancelIdleCallback: int => unit = "cancelIdleCallback"
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Window/getSelection)
 */
-external getSelection: unit => selection = "getSelection"
+external getSelection: unit => null<selection> = "getSelection"