Fix incorrect id fields in empty nested results with MongoDB

`id` was returned instead of `_id` because of the wrong method being
called. This bug was previously masked by serialization logic silently
filtering out unknown fields before `.zip()`, making the rest of the
fields mismatch the corresponding values, which wasn't a problem in this
case because there are no values in an empty result.

query-engine/query-structure/src/record.rs

@@ -49,7 +49,7 @@ impl ManyRecords {
     pub fn empty(selected_fields: &FieldSelection) -> Self {
         Self {
             records: Vec::new(),
-            field_names: selected_fields.prisma_names().collect(),
+            field_names: selected_fields.db_names().collect(),
         }
     }