crypto: algif_skcipher - Add key check exception for cipher_null

commit 6e8d8ecf438792ecf7a3207488fb4eebc4edb040 upstream.

This patch adds an exception to the key check so that cipher_null
users may continue to use algif_skcipher without setting a key.

Cc: stable@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>