Change the semantics of OSSL_LIB_CTX_set0_default() NULL handling

Change the semantics of OSSL_LIB_CTX_set0_default() NULL handling

Change things so that passing NULL to OSSL_LIB_CTX_set0_default() means
keep the current library context unchanged.

This has the advantage of simplifying error handling, e.g. you can call
OSSL_LIB_CTX_set0_default in an error/finalisation block safe in the
knowledge the if the "prevctx" was never set then it will be a no-op (like
calling a "free" function with NULL).

Fixes #14593

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14890)