Return NULL from ossl_lib_ctx_get_concrete() when it is uninitialized

Return NULL from ossl_lib_ctx_get_concrete() when it is uninitialized

When default_context_inited is set to false we return NULL instead of
the global default context.

Fixes #25442

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26319)

(cherry picked from commit dfce0d7418d6d5b54d74fa80fc50392f00270c53)