hkdf: when HMAC key is all zeros, still set a valid key length

hkdf: when HMAC key is all zeros, still set a valid key length

By itself, this is no change in any computation. However, this will
unlock enforcing minimum key lengths for NIST and FIPS 140-3
requirements.

Also reading RFC8448 and RFC5869, this seems to be strictly correct
too.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@surgut.co.uk>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24204)