### What changes were proposed in this pull request?
Convert `NULL` elements of maps, structs and arrays to the `"null"` string while converting maps/struct/array values to strings. The SQL config `spark.sql.legacy.omitNestedNullInCast.enabled` controls the behaviour. When it is `true`, `NULL` elements of structs/maps/arrays will be omitted otherwise, when it is `false`, `NULL` elements will be converted to `"null"`.
### Why are the changes needed?
1. It is impossible to distinguish empty string and null, for instance:
```scala
scala> Seq(Seq(""), Seq(null)).toDF().show
+-----+
|value|
+-----+
| []|
| []|
+-----+
```
2. Inconsistent NULL conversions for top-level values and nested columns, for instance:
```scala
scala> sql("select named_struct('c', null), null").show
+---------------------+----+
|named_struct(c, NULL)|NULL|
+---------------------+----+
| []|null|
+---------------------+----+
```
3. `.show()` is different from conversions to Hive strings, and as a consequence its output is different from `spark-sql` (sql tests):
```sql
spark-sql> select named_struct('c', null) as struct;
{"c":null}
```
```scala
scala> sql("select named_struct('c', null) as struct").show
+------+
|struct|
+------+
| []|
+------+
```
4. It is impossible to distinguish empty struct/array from struct/array with null in the current implementation:
```scala
scala> Seq[Seq[String]](Seq(), Seq(null)).toDF.show()
+-----+
|value|
+-----+
| []|
| []|
+-----+
```
### Does this PR introduce _any_ user-facing change?
Yes, before:
```scala
scala> Seq(Seq(""), Seq(null)).toDF().show
+-----+
|value|
+-----+
| []|
| []|
+-----+
```
After:
```scala
scala> Seq(Seq(""), Seq(null)).toDF().show
+------+
| value|
+------+
| []|
|[null]|
+------+
```
### How was this patch tested?
By existing test suite `CastSuite`.
Closes#29311 from MaxGekk/nested-null-to-string.
Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
3a437ed22b