dsl_util: use .ok() to drop unprintable error

I used .map_err(|_: Vec<_>|) to clarify that the original data is returned as
an error (so it can't be .unwrap()-ed.) However, it can be said that the error
detail isn't important and .map_err() is too verbose.

lib/src/dsl_util.rs

@@ -97,7 +97,7 @@ impl<'i, T> FunctionCallNode<'i, T> {
             optional.resize(M, None);
             Ok((
                 required.try_into().unwrap(),
-                optional.try_into().map_err(|_: Vec<_>| ()).unwrap(),
+                optional.try_into().ok().unwrap(),
             ))
         } else {
             let (min, max) = count_range.into_inner();