Add a null provider which implements no algorithms.

Add a null provider which implements no algorithms.

By loading the null provider into the default context, it is possible
to verify that it is not accidentally being used.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11488)