Allow $tag to be null. This is the value that openssl_encrypt()
sets it to for non-AEAD ciphers, so we should also accept this
as an input to openssl_decrypt().
Prior to PHP 8.1, null was accepted in weak mode due to the special
treatment of null arguments to internal functions.
openssl_encrypt() currently throws a warning if the $tag out
parameter is passed for a non-authenticated cipher. This violates
the principle that a function should behave the same if a parameter
is not passed, and if the default value is passed for the parameter.
I believe this warning should simply be dropped and the $tag be
populated with null, as is already the case. Otherwise, it is not
possible to use openssl_encrypt() in generic wrapper APIs, that are
compatible with both authenticated and non-authenticated encryption.
Closes GH-6333.
