Issue #3177765 by klausi: ListInterface::first() return value is documented wrong

(cherry picked from commit 7655860e)

core/lib/Drupal/Core/TypedData/ListInterface.php

@@ -83,8 +83,8 @@ public function set($index, $value);
   /**
    * Returns the first item in this list.
    *
-   * @return \Drupal\Core\TypedData\TypedDataInterface
-   *   The first item in this list.
+   * @return \Drupal\Core\TypedData\TypedDataInterface|null
+   *   The first item in this list, or NULL if there are no items.
    *
    * @throws \Drupal\Core\TypedData\Exception\MissingDataException
    *   If the complex data structure is unset and no item can be created.