 BUG: allow None as upcasted_na value 
In geopandas I use `dtype.na_value = None`, but this gives problems in `merge` (in `JoinUnit.get_reindexed_values`, `upcasted_na` of None is handled specifically).

The first commit works, but is not very nice, so in the second commit I attempted to just return `values` instead of `get_values`