Fix remaining implicit marking of parameters as nullable (PHP 8.4)

[steffenweber]

Webklex\PHPIMAP\Decoder\MessageDecoder::decode(): Implicitly marking parameter $encoding as nullable is deprecated, the explicit nullable type must be used instead in vendor/webklex/php-imap/src/Decoder/MessageDecoder.php:26

Found more occurences using find . -name *.php -exec php -l {} \; and fixed them all.

[Webklex]

Hi @steffenweber,
many thanks for your pull request and your patience <3

Best regards & happy coding,