Support writing NULL partition values to the Hive default partition directory #26749

Follow-up from #26310 (review):

INSERT/COPY ... PARTITIONED BY still reject NULL partition values (NULL values are not supported for partition column), so a NULL written by DataFusion can't round-trip through the placeholder. This fails on main too, so it isn't a regression, but now that the column is nullable it is worth an issue.

Partitioned writes could write NULL partition values to the configured datafusion.execution.listing_table_hive_default_partition_name directory, which reads back as NULL after #26310.