Fix missing NULL check in prov_config_test

Fix missing NULL check in prov_config_test

coverity-1596500 caught a missing null check.  We should never hit it as
the test harness always sets the environment variable, but lets add the
check for safety

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24208)