EVP_PKEY_new_raw_private_key: Allow zero length keys

Allocate at least one byte to distinguish a zero length key
from an unset key.

Fixes #15632

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15643)

test/evp_extra_test.c

diff --git a/test/evp_extra_test.c b/test/evp_extra_test.c
index 61f6b4c..33bb698 100644
--- a/test/evp_extra_test.c
+++ b/test/evp_extra_test.c
@@ -1681,6 +1681,9 @@
 } keys[] = {
     {
         EVP_PKEY_HMAC, "0123456789", NULL
+    },
+    {
+        EVP_PKEY_HMAC, "", NULL
 #ifndef OPENSSL_NO_POLY1305
     }, {
         EVP_PKEY_POLY1305, "01234567890123456789012345678901", NULL