crypto: algif_skcipher - Add key check exception for cipher_null

commit 6e8d8ecf 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.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>