crypto: testmgr - add empty test vectors for null ciphers

Without these, kernel log shows:
[    5.984881] alg: No test for cipher_null (cipher_null-generic)
[    5.985096] alg: No test for ecb(cipher_null) (ecb-cipher_null)
[    5.985170] alg: No test for compress_null (compress_null-generic)
[    5.985297] alg: No test for digest_null (digest_null-generic)

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>