Fix fips cleanup of master key

Fix fips cleanup of master key

With the conversion to a fixed array, we can no longer just clean the
top level sparse array, as we don't enjoy the NULL check within the
sa_doall loop.

Add a NULL check in the fips cleanup routine to make up for this

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27794)