Fix implicit nullable parameter deprecation for PHP 8.4

Add explicit nullable type hints (?string) to parameters that have NULL as default value in SmartyPants class methods:

• modifierSmartypants(): $attr parameter
• process(): $attr and $ctx parameters
• smartQuotes(): $attr and $ctx parameters
• smartDashes(): $attr and $ctx parameters
• smartEllipses(): $attr and $ctx parameters

Closes #3569369