[SPARK-31091] Revert SPARK-24640 Return NULL from size(NULL) by default

docs/sql-migration-guide.md

@@ -217,8 +217,6 @@ license: |
     - `now` - current query start time
   For example `SELECT timestamp 'tomorrow';`.
 
-  - Since Spark 3.0, the `size` function returns `NULL` for the `NULL` input. In Spark version 2.4 and earlier, this function gives `-1` for the same input. To restore the behavior before Spark 3.0, you can set `spark.sql.legacy.sizeOfNull` to `true`.
-
   - Since Spark 3.0, when the `array`/`map` function is called without any parameters, it returns an empty collection with `NullType` as element type. In Spark version 2.4 and earlier, it returns an empty collection with `StringType` as element type. To restore the behavior before Spark 3.0, you can set `spark.sql.legacy.createEmptyCollectionUsingStringType` to `true`.
 
   - Since Spark 3.0, the interval literal syntax does not allow multiple from-to units anymore. For example, `SELECT INTERVAL '1-1' YEAR TO MONTH '2-2' YEAR TO MONTH'` throws parser exception.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala

@@ -79,7 +79,7 @@ trait BinaryArrayExpressionWithImplicitCast extends BinaryExpression
     _FUNC_(expr) - Returns the size of an array or a map.
     The function returns -1 if its input is null and spark.sql.legacy.sizeOfNull is set to true.
     If spark.sql.legacy.sizeOfNull is set to false, the function returns null for null input.
-    By default, the spark.sql.legacy.sizeOfNull parameter is set to false.
+    By default, the spark.sql.legacy.sizeOfNull parameter is set to true.
   """,
   examples = """
     Examples:
@@ -88,7 +88,7 @@ trait BinaryArrayExpressionWithImplicitCast extends BinaryExpression
       > SELECT _FUNC_(map('a', 1, 'b', 2));
        2
       > SELECT _FUNC_(NULL);
-       NULL
+       -1
   """)
 case class Size(child: Expression, legacySizeOfNull: Boolean)
   extends UnaryExpression with ExpectsInputTypes {

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -2157,7 +2157,7 @@ object SQLConf {
       "The size function returns null for null input if the flag is disabled.")
     .version("")
     .booleanConf
-    .createWithDefault(false)
+    .createWithDefault(true)
 
   val LEGACY_REPLACE_DATABRICKS_SPARK_AVRO_ENABLED =
     buildConf("spark.sql.legacy.replaceDatabricksSparkAvro.enabled")