crypto: algif_aead - fix AIO handling of zero buffer

Handle the case when the caller provided a zero buffer to
sendmsg/sendpage. Such scenario is legal for AEAD ciphers when no
plaintext / ciphertext and no AAD is provided and the caller only
requests the generation of the tag value.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>