Allow our *_gen_cleanup functions to tolerate a NULL ctx

Allow our *_gen_cleanup functions to tolerate a NULL ctx

Our *_gen_cleanup functions are essentially "free" functions. Our
free functions tolerate NULL being passed. We are being inconsistent with
our *_gen_cleanup functions. Some of them tolerate NULL and others do not.

We should consistently tolerate NULL.

See also #27795

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27807)