Warnings in util.Secret with PHP 8.1

[thekid]

Using new Secret(null), the following warnings are raised in PHP 8.1+:

• [x] sodium_crypto_secretbox(): Passing null to parameter 1 ($message) of type string is deprecated" in ::sodium_crypto_secretbox() (Secret.class.php, line 66, occured once)
• [x] openssl_encrypt(): Passing null to parameter 1 ($data) of type string is deprecated" in ::openssl_encrypt() (Secret.class.php, line 79, occured once)
• [x] strlen(): Passing null to parameter 1 ($string) of type string is deprecated" in ::strlen() (Secret.class.php, line 138, occured once)

See https://github.com/xp-forge/web-auth/runs/2106925404#step:10:47

[thekid]

:shipit: Fix released in https://github.com/xp-framework/core/releases/tag/v10.8.1