If not set, the table is partitioned by pseudo column '_PARTITIONTIME'; if set, the table is partitioned by this field. The field must be a top-level TIMESTAMP or DATE field. Its mode must be NULLABLE or REQUIRED. A wrapper is used here because an empty string is an invalid value.
The only type supported is DAY, which will generate one partition per day. Providing an empty string used to cause an error, but in OnePlatform the field will be treated as unset.