[PHP 8.4] Fixes for implicit nullability deprecation

Fixes all issues that emit deprecation notices on PHP 8.4 for implicit nullable parameter type declarations.

See:
 - [RFC](https://wiki.php.net/rfc/deprecate-implicitly-nullable-types)
 - [PHP 8.4: Implicitly nullable parameter declarations deprecated](https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated)

Author: Ayesh

Parsedown.php

@@ -570,7 +570,7 @@ protected function blockHeader($Line)
     #
     # List
 
-    protected function blockList($Line, array $CurrentBlock = null)
+    protected function blockList($Line, ?array $CurrentBlock = null)
     {
         list($name, $pattern) = $Line['text'][0] <= '-' ? array('ul', '[*+-]') : array('ol', '[0-9]{1,9}+[.\)]');
 
@@ -807,7 +807,7 @@ protected function blockRule($Line)
     #
     # Setext
 
-    protected function blockSetextHeader($Line, array $Block = null)
+    protected function blockSetextHeader($Line, ?array $Block = null)
     {
         if ( ! isset($Block) or $Block['type'] !== 'Paragraph' or isset($Block['interrupted']))
         {
@@ -893,7 +893,7 @@ protected function blockReference($Line)
     #
     # Table
 
-    protected function blockTable($Line, array $Block = null)
+    protected function blockTable($Line, ?array $Block = null)
     {
         if ( ! isset($Block) or $Block['type'] !== 'Paragraph' or isset($Block['interrupted']))
         {