Add support for PHP8's Nullsafe operator

[heldchen]

What is this feature about (expected vs actual behaviour)?

PHP 8 added the Nullsafe operator (https://wiki.php.net/rfc/nullsafe_operator) - unfortunately none of the query functions currently return null. maybe you'll find the Nullsafe operator useful as well and might consider adding corresponding functions like findOrNull() & findOneOrNull()? thanks :-)

that would allow performant chaining of complex queries:

$text = $dom->findOneOrNull('something`)?->findOneOrNull('somethingelse`)?->findOneOrNull('somethingelse`)?->text();

accidentially this already partially works (due to #63) but I feel like having proper null returning functions might be more performant as the Nullsafe operator can abort early on.

Does it take minutes, hours or days to fix?

minutes

[voku]

Please go for it, create a pull request, and we can take a look at it. 👍