Remove implicitly nullable parameter declarations.

[mateusz-kaczmarek-paysera]

To get rid of PHP 8.4 deprecations, the code was adjusted.

https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

[mateusz-kaczmarek-paysera]

Updated Github CI:

• for php 7.1 which have composer v1 using env variable config to avoid security errors COMPOSER_AUDIT_BLOCK_INSECURE=0
• for other versions using syntax for composer v2 composer config --global audit.ignore "PKSA-w2tw-kmfg-rt9s"