Adding missing NULL pointer check

Adding missing NULL pointer check

CLA: trivial
In the provider store API, it is not necessary to provide both open and
attach method at the same time and providing at least one of them is
enough. Adding some null pointer checks to prevent exceptions in case
of not providing both methods at the same time.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23703)