Test NULL client CA lists.

The preceding client CA bug is actually almost unreachable since the
list is initialized to a non-NULL empty list. But if one tries hard
enough, a NULL one is possible.

Change-Id: I49e69511bf65b0178c4e0acdb887f8ba7d85faff
Reviewed-on: https://boringssl-review.googlesource.com/8769
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/ssl/test/test_config.cc b/ssl/test/test_config.cc
index 32b91e9..1c3302e 100644
--- a/ssl/test/test_config.cc
+++ b/ssl/test/test_config.cc
@@ -102,6 +102,7 @@
   { "-use-sparse-dh-prime", &TestConfig::use_sparse_dh_prime },
   { "-use-old-client-cert-callback",
     &TestConfig::use_old_client_cert_callback },
+  { "-use-null-client-ca-list", &TestConfig::use_null_client_ca_list },
 };
 
 const Flag<std::string> kStringFlags[] = {