[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)

Parsedown.php

@@ -712,7 +712,7 @@ protected function blockRule($Line)
     #
     # Setext
 
-    protected function blockSetextHeader($Line, array $Block = null)
+    protected function blockSetextHeader($Line, ?array $Block = null)
     {
         if ( ! isset($Block) or isset($Block['type']) or isset($Block['interrupted']))
         {
@@ -850,7 +850,7 @@ protected function blockReference($Line)
     #
     # Table
 
-    protected function blockTable($Line, array $Block = null)
+    protected function blockTable($Line, ?array $Block = null)
     {
         if ( ! isset($Block) or isset($Block['type']) or isset($Block['interrupted']))
         {