Allow keygen after dup of minimal PKEY ctx

Allow keygen after dup of minimal PKEY ctx

It should be possible to repeatedly duplicate a PKEY CTX created via
EVP_PKEY_CTX_new_from_name() that has not yet been assigned an
"operation" (e.g. via EVP_PKEY_CTX_keygen_init()), and then perform
keygen_init() and keygen() on the duplicated copies.

When the operation is not yet set, all that's needed is to not try to
use the key if one isn't set yet.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27662)