Make sure that if a buffer is allowed to be empty, a null pointer is
accepted if the buffer length is 0. This was already the case for most
but not all arguments to mbedtls_cipher_auth_{en,de}crypt{,_ext}.
Make sure to pass NULL for an empty buffer in the tests.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>