Fix DocBlock in Style (#3200)

There is a DocBlock `@var ?array<string, array>` which causes PhpDocumentor to fail parsing Style/Style, so the online API documentation for that class is missing. Changing to `@var null|array<string, array>` fixes the problem.

src/PhpSpreadsheet/Style/Style.php

@@ -78,7 +78,7 @@ class Style extends Supervisor
      * @see Style::applyFromArray()
      * @see Style::getHashCode()
      *
-     * @var ?array<string, array>
+     * @var null|array<string, array>
      */
     private static $cachedStyles;