Get null values for for the nested field partition column

[ConeyLiu]

Code to reproduce:

scala> case class Data(id: Long, name: String)
scala> case class Item(id: Long, data: Data)

scala> spark.sql("create table local.db.test_table(id long, data struct<id: long, name: string>) using iceberg partitioned by (identity(data.name))").show()
scala> spark.range(5).map(i => Item(i, Data(i, s"${i}"))).writeTo("local.db.test_table").append()

scala> spark.sql("select data.name from local.db.test_table").show()
+----+
|name|
+----+
|null|
|null|
|null|
|null|
|null|
+----+

[kbendick]

Linking this somewhat related issue as it's mentioned in the tests for this issues PR #4627 #4604

[github-actions]

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[ConeyLiu]

Hi @kbendick @szehon-ho @nastra, hope your guys could help take a look at this issue again, thanks a lot.