fix: ossl_digest_get_approved_nid() returns NID_undef on invalid digest

We checked using 'md_nid < 0', which is faulty.

Impact: DSA and ECDSA signature provider implementations

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24992)