Message::setHTML will produce deprecation warnings in PHP 8 if null is passed

[vortrixs]

mail/src/Message.php

Lines 347 to 351 in 92f2ce0

	public function setHTML(?string $html): void
	{
	if (preg_match('//u', $html) !== 1) {
	throw new InvalidArgumentException("message body contains an invalid UTF-8 byte sequence");
	}

This will trigger a deprecation warning if null is passed because preg_match only accepts strings as of PHP 8.

Error: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated