added null ? to type-hints in method arguments

[petrdobr]

Problem this PR solves

I tried to use this library in my project (PHP 8.4) to hyphenate words. Hyphenation works fine, but when I run scripts I get a lot of messages of this type

Deprecated: PHP_Typography\Fixes\Token_Fixes\Wrap_Emails_Fix::apply(): Implicitly marking parameter $textnode as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/vendor/mundschenk-at/php-typography/src/fixes/token-fixes/class-wrap-emails-fix.php on line 90

To fix this behavior I added null ? symbol in every place I found which can cause such deprecation message, .i.e. places with null default value for arguments, like

function (?string $a = null) {}

This should help the library work without any non-relevant messages in stdOut