[SPARK-33134][SQL][3.0] Return partial results only for root JSON objects

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to restrict the partial result feature only by root JSON objects. JSON datasource as well as from_json() will return null for malformed nested JSON objects.

Why are the changes needed?

1. To not raise exception to users in the PERMISSIVE mode
2. To fix a regression and to have the same behavior as Spark 2.4.x has
3. Current implementation of partial result is supposed to work only for root (top-level) JSON objects, and not tested for bad nested complex JSON fields.

Does this PR introduce any user-facing change?

Yes. Before the changes, the code below:

    val pokerhand_raw = Seq("""[{"cards": [19], "playerId": 123456}]""").toDF("events")
    val event = new StructType().add("playerId", LongType).add("cards", ArrayType(new StructType().add("id", LongType).add("rank", StringType)))
    val pokerhand_events = pokerhand_raw.select(from_json($"events", ArrayType(event)).as("event"))
    pokerhand_events.show

throws the exception even in the default PERMISSIVE mode:

java.lang.ClassCastException: java.lang.Long cannot be cast to org.apache.spark.sql.catalyst.util.ArrayData
  at org.apache.spark.sql.catalyst.expressions.BaseGenericInternalRow.getArray(rows.scala:48)
  at org.apache.spark.sql.catalyst.expressions.BaseGenericInternalRow.getArray$(rows.scala:48)
  at org.apache.spark.sql.catalyst.expressions.GenericInternalRow.getArray(rows.scala:195)

After the changes:

+-----+
|event|
+-----+
| null|
+-----+

How was this patch tested?

Added a test to JsonFunctionsSuite.

[HyukjinKwon]

Merged to branch-3.0.