Update phpdoc for widget->getId (#18295)

The private variable `$_id`  is only set when `$autogenerate` is true, else it stays null, and will be returned by the function.

framework/base/Widget.php

@@ -156,7 +156,7 @@ class Widget extends Component implements ViewContextInterface
     /**
      * Returns the ID of the widget.
      * @param bool $autoGenerate whether to generate an ID if it is not set previously
-     * @return string ID of the widget.
+     * @return string|null ID of the widget.
      */
     public function getId($autoGenerate = true)
     {