Issue #3444028 by andypost: [8.4] Fix implicitly nullable type declarations in core/includes

(cherry picked from commit dfcc8e8b)

core/includes/form.inc

@@ -868,7 +868,7 @@ function _batch_append_set(&$batch, $batch_set) {
  *   redirect response was returned by the 'finished' callback, the user will
  *   be redirected to the page that started the batch. Any query arguments will
  *   be automatically persisted.
- * @param \Drupal\Core\Url $url
+ * @param \Drupal\Core\Url|null $url
  *   (optional) URL of the batch processing page. Should only be used for
  *   separate scripts like update.php.
  * @param $redirect_callback
@@ -878,7 +878,7 @@ function _batch_append_set(&$batch, $batch_set) {
  * @return \Symfony\Component\HttpFoundation\RedirectResponse|null
  *   A redirect response if the batch is progressive. No return value otherwise.
  */
-function batch_process($redirect = NULL, Url $url = NULL, $redirect_callback = NULL) {
+function batch_process($redirect = NULL, ?Url $url = NULL, $redirect_callback = NULL) {
   $batch =& batch_get();
 
   if (isset($batch)) {

core/includes/install.core.inc

@@ -97,13 +97,13 @@
  *   page request (optimized for the command line) and not send any output
  *   intended for the web browser. See install_state_defaults() for a list of
  *   elements that are allowed to appear in this array.
- * @param callable $callback
+ * @param callable|null $callback
  *   (optional) A callback to allow command line processes to update a progress
  *   bar. The callback is passed the $install_state variable.
  *
  * @see install_state_defaults()
  */
-function install_drupal($class_loader, $settings = [], callable $callback = NULL) {
+function install_drupal($class_loader, $settings = [], ?callable $callback = NULL) {
   global $install_state;
   // Initialize the installation state with the settings that were passed in,
   // as well as a boolean indicating whether or not this is an interactive
@@ -548,14 +548,14 @@ function install_begin_request($class_loader, &$install_state) {
  * @param $install_state
  *   An array of information about the current installation state. This is
  *   passed along to each task, so it can be modified if necessary.
- * @param callable $callback
+ * @param callable|null $callback
  *   (optional) A callback to allow command line processes to update a progress
  *   bar. The callback is passed the $install_state variable.
  *
  * @return array|null
  *   HTML output from the last completed task.
  */
-function install_run_tasks(&$install_state, callable $callback = NULL) {
+function install_run_tasks(&$install_state, ?callable $callback = NULL) {
   do {
     // Obtain a list of tasks to perform. The list of tasks itself can be
     // dynamic (e.g., some might be defined by the installation profile,