MaxGekk opened a new pull request #29311:
URL: https://github.com/apache/spark/pull/29311
### 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.
### 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|
+------+
| []|
+------+
```
### 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`.
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