kc/configuration: use system trust when no truststore is provided

When no trust store (CA certificate chain) is provided in kafka client configuration the logic
should default to system embedded trust store. Previously the code path
was missing forcing users to set the trust store manually.

Signed-off-by: Michał Maślanka <michal@redpanda.com>
(cherry picked from commit 907125a)

Author: mmaslankaprv

src/v/kafka/client/configuration.cc

@@ -343,7 +343,10 @@ tls_configuration::build_credentials() const {
                 truststore_str, ss::tls::x509_crt_format::PEM);
               return ss::now();
           });
+    } else {
+        co_await builder.set_system_trust();
     }
+
     if (k_store) {
         co_await ss::visit(
           *k_store,