OBJ_nid2obj(): Return UNDEF object instead of NULL for NID_undef

Fixes a regression from 3.0 from the obj creation refactoring.

Fixes #20555

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20556)