wasm: fix panic on empty Struct expression

compiler/gen_wasm/src/backend.rs

@@ -936,11 +936,13 @@ impl<'a> WasmBackend<'a> {
                 }
                 _ => internal_error!("Cannot create struct {:?} with storage {:?}", sym, storage),
             };
-        } else {
+        } else if !fields.is_empty() {
             // Struct expression but not Struct layout => single element. Copy it.
             let field_storage = self.storage.get(&fields[0]).to_owned();
             self.storage
                 .clone_value(&mut self.code_builder, storage, &field_storage, fields[0]);
+        } else {
+            // Empty record. Nothing to do.
         }
     }