Short-circuit `peer_cert` if we're insecure or unauthenticated

src/ruby/ext/grpc/rb_call.c

@@ -219,11 +219,14 @@ static VALUE grpc_rb_call_get_peer_cert(VALUE self) {
   grpc_call *call = NULL;
   VALUE res = Qnil;
   grpc_auth_context *ctx = NULL;
-  // char *peer_cert = NULL;
   TypedData_Get_Struct(self, grpc_call, &grpc_call_data_type, call);
 
   ctx = grpc_call_auth_context(call);
 
+  if (!ctx || !grpc_auth_context_peer_is_authenticated(ctx)) {
+    return Qnil;
+  }
+
   grpc_auth_property_iterator it =
       grpc_auth_context_find_properties_by_name(ctx, GRPC_X509_PEM_CERT_PROPERTY_NAME);
   const grpc_auth_property *prop = grpc_auth_property_iterator_next(&it);
@@ -233,6 +236,8 @@ static VALUE grpc_rb_call_get_peer_cert(VALUE self) {
 
   res = rb_str_new2(prop->value);
 
+  grpc_auth_context_release(ctx);
+
   return res;
 }