EVP_PBE_scrypt() handles salt=NULL as salt=""

Modify EVP_PBE_scrypt() to maintain OpenSSL 1.1.1 behavior: salt=NULL
is now handled as salt="" (and saltlen=0).

Commit 5a285add

 changed the behavior
of EVP_PBE_scrypt(salt=NULL). Previously, salt=NULL was accepted, but
the function now fails with KDF_R_MISSING_SALT.

CLA: trivial

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8483)