Allow 0 length plaintext and aad for aes-siv

Test vectors taken from wycheproof:

https://github.com/C2SP/wycheproof/blob/master/testvectors/aes_siv_cmac_test.json

Fixes #26580

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26658)

crypto/modes/siv128.c

@@ -291,7 +291,7 @@ int ossl_siv128_encrypt(SIV128_CONTEXT *ctx,
     if (!siv128_do_encrypt(ctx->cipher_ctx, out, in, len, &q))
         return 0;
     ctx->final_ret = 0;
-    return len;
+    return 1;
 }
 
 /*
@@ -327,7 +327,7 @@ int ossl_siv128_decrypt(SIV128_CONTEXT *ctx,
         return 0;
     }
     ctx->final_ret = 0;
-    return len;
+    return 1;
 }
 
 /*

providers/implementations/ciphers/cipher_aes_siv.c

@@ -118,19 +118,10 @@ static int siv_cipher(void *vctx, unsigned char *out, size_t *outl,
     if (!ossl_prov_is_running())
         return 0;
 
-    /* Ignore just empty encryption/decryption call and not AAD. */
-    if (out != NULL) {
-        if (inl == 0) {
-            if (outl != NULL)
-                *outl = 0;
-            return 1;
-        }
-
-        if (outsize < inl) {
+    if (out != NULL && outsize < inl) {
         ERR_raise(ERR_LIB_PROV, PROV_R_OUTPUT_BUFFER_TOO_SMALL);
         return 0;
     }
-    }
 
     if (ctx->hw->cipher(ctx, out, in, inl) <= 0)
         return 0;