[SPARK-31171][SQL] size(null) should return null under ansi mode

### What changes were proposed in this pull request?

Make `size(null)` return null under ANSI mode, regardless of the `spark.sql.legacy.sizeOfNull` config.

### Why are the changes needed?

In apache#27834, we change the result of `size(null)` to be -1 to match the 2.4 behavior and avoid breaking changes.

However, it's true that the "return -1" behavior is error-prone when being used with aggregate functions. The current ANSI mode controls a bunch of "better behaviors" like failing on overflow. We don't enable these "better behaviors" by default because they are too breaking. The "return null" behavior of `size(null)` is a good fit of the ANSI mode.

### Does this PR introduce any user-facing change?

No as ANSI mode is off by default.

### How was this patch tested?

new tests

Closes apache#27936 from cloud-fan/null.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: cloud-fan

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

@@ -2154,8 +2154,8 @@ object SQLConf {
 
   val LEGACY_SIZE_OF_NULL = buildConf("spark.sql.legacy.sizeOfNull")
     .internal()
-    .doc("If it is set to true, size of null returns -1. This behavior was inherited from Hive. " +
-      "The size function returns null for null input if the flag is disabled.")
+    .doc(s"If it is set to false, or ${ANSI_ENABLED.key} is true, then size of null returns " +
+      "null. Otherwise, it returns -1, which was inherited from Hive.")
     .version("2.4.0")
     .booleanConf
     .createWithDefault(true)
@@ -3014,7 +3014,10 @@ class SQLConf extends Serializable with Logging {
 
   def csvColumnPruning: Boolean = getConf(SQLConf.CSV_PARSER_COLUMN_PRUNING)
 
-  def legacySizeOfNull: Boolean = getConf(SQLConf.LEGACY_SIZE_OF_NULL)
+  def legacySizeOfNull: Boolean = {
+    // size(null) should return null under ansi mode.
+    getConf(SQLConf.LEGACY_SIZE_OF_NULL) && !getConf(ANSI_ENABLED)
+  }
 
   def isReplEagerEvalEnabled: Boolean = getConf(SQLConf.REPL_EAGER_EVAL_ENABLED)
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala

@@ -74,6 +74,12 @@ class CollectionExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper
     withSQLConf(SQLConf.LEGACY_SIZE_OF_NULL.key -> "false") {
       testSize(sizeOfNull = null)
     }
+    // size(null) should return null under ansi mode.
+    withSQLConf(
+      SQLConf.LEGACY_SIZE_OF_NULL.key -> "true",
+      SQLConf.ANSI_ENABLED.key -> "true") {
+      testSize(sizeOfNull = null)
+    }
   }
 
   test("MapKeys/MapValues") {

sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala

@@ -490,6 +490,12 @@ class DataFrameFunctionsSuite extends QueryTest with SharedSparkSession {
     withSQLConf(SQLConf.LEGACY_SIZE_OF_NULL.key -> "false") {
       testSizeOfArray(sizeOfNull = null)
     }
+    // size(null) should return null under ansi mode.
+    withSQLConf(
+      SQLConf.LEGACY_SIZE_OF_NULL.key -> "true",
+      SQLConf.ANSI_ENABLED.key -> "true") {
+      testSizeOfArray(sizeOfNull = null)
+    }
   }
 
   test("dataframe arrays_zip function") {
@@ -569,6 +575,12 @@ class DataFrameFunctionsSuite extends QueryTest with SharedSparkSession {
     withSQLConf(SQLConf.LEGACY_SIZE_OF_NULL.key -> "false") {
       testSizeOfMap(sizeOfNull = null)
     }
+    // size(null) should return null under ansi mode.
+    withSQLConf(
+      SQLConf.LEGACY_SIZE_OF_NULL.key -> "true",
+      SQLConf.ANSI_ENABLED.key -> "true") {
+      testSizeOfMap(sizeOfNull = null)
+    }
   }
 
   test("map_keys/map_values function") {