### What changes were proposed in this pull request?
This PR proposes to throw exception by default when user use untyped UDF(a.k.a `org.apache.spark.sql.functions.udf(AnyRef, DataType)`).
And user could still use it by setting `spark.sql.legacy.useUnTypedUdf.enabled` to `true`.
### Why are the changes needed?
According to #23498, since Spark 3.0, the untyped UDF will return the default value of the Java type if the input value is null. For example, `val f = udf((x: Int) => x, IntegerType)`, `f($"x")` will return 0 in Spark 3.0 but null in Spark 2.4. And the behavior change is introduced due to Spark3.0 is built with Scala 2.12 by default.
As a result, this might change data silently and may cause correctness issue if user still expect `null` in some cases. Thus, we'd better to encourage user to use typed UDF to avoid this problem.
### Does this PR introduce any user-facing change?
Yeah. User will hit exception now when use untyped UDF.
### How was this patch tested?
Added test and updated some tests.
Closes#27488 from Ngone51/spark_26580_followup.
Lead-authored-by: yi.wu <yi.wu@databricks.com>
Co-authored-by: wuyi <yi.wu@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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