fix: PHP 8.1+ null parameter deprecation warnings in Generator.php

[sysmonf]

Add null coalescing operator (?? '') to prevent deprecation warnings on PHP 8.1-8.4 and TypeError on PHP 8.5+ when null is passed to built-in string functions.

Of note, htmlpurifier was ran on tens of thousands of html code using PHP 8.5, and this is all the deprecation errors we noted on PHP 8.5.

Changes:

• generateAttributes(): strpos(), strcspn(), strlen() with null $value
• escape(): htmlspecialchars() with null $string
• Flash compat: $token->attr['name'] / $token->attr['value'] may not exist