Properly initialize AEAD cipher flags in OpenSSL backend

[jordikroon]

Fixes #20851

Add support for AEAD ciphers like AES-SIV by detecting and initializing AEAD flags during cipher mode loading.
Includes test case for AES-256-SIV encryption/decryption roundtrip.

[jordikroon]

Please review commits 1 by 1 since I am not sure if the change from string $aad = "" to ?string $aad = "" is acceptable.
The first commit only fixes SIV Synthetic Initialization Vector where it generates its own IV. It was not respected.

The second commit allows AAD to be null since it behaves differently than when only an empty string is given. The matches other implementations like cryptography in python.