:bug: allow nans in ordinal types & add missing fields to `combine_variables_metadata`

[Marigold]

Fix for https://github.com/owid/etl/issues/2636

When working on this, I noticed that we don't combine type and sort (e.g. dropna() loses information about ordinal types). I fixed those fields (I haven't fixed all possible edge cases as it is rarely used and typically in a grapher step) and added unit test in case we add more fields to remind us about adding them.