Give a readable error in mtr if resolve_at_variable fails

mysql-test/lib/My/ConfigFactory.pm

@@ -347,9 +347,16 @@ sub resolve_at_variable {
     or croak "There is no group named '$group_name' that ",
       "can be used to resolve '$option_name' for test '$self->{testname}'";
 
-    my $value= $from_group->value($option_name) || '';
+    my $value= $from_group->value($option_name);
+    if (!defined($value))
+    {
+      ::mtr_verbose("group: $group_name  option_name: $option_name is undefined");
+    }
+    else
+    {
       $res .= $before.$value;
     }
+  }
   $res .= $after;
 
   $option->{value}= $res;