Use indirection for doc.

src/api/optimize.rs

@@ -63,7 +63,11 @@ impl Engine {
         );
 
         #[cfg(feature = "metadata")]
-        _new_ast.set_doc(std::mem::take(ast.doc_mut()));
+        if let Some(doc) = ast.doc_mut() {
+            _new_ast.set_doc(std::mem::take(doc));
+        } else {
+            _new_ast.clear_doc();
+        }
 
         _new_ast
     }