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

### What changes were proposed in this pull request?
Set the default value of the `spark.sql.legacy.sizeOfNull` config to `false`. That changes behavior of the `size()` function for `NULL`. The function will return `NULL` for `NULL` instead of `-1`.

### Why are the changes needed?
There is the agreement in the PR #21598 (comment) to change behavior in Spark 3.0.

### Does this PR introduce any user-facing change?
Yes.
Before:
```sql
spark-sql> select size(NULL);
-1
```
After:
```sql
spark-sql> select size(NULL);
NULL
```

### How was this patch tested?
By the `check outputs of expression examples` test in `SQLQuerySuite` which runs expression examples.

Closes #26051 from MaxGekk/sizeof-null-returns-null.

Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

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 true.
+    By default, the spark.sql.legacy.sizeOfNull parameter is set to false.
   """,
   examples = """
     Examples:
@@ -88,7 +88,7 @@ trait BinaryArrayExpressionWithImplicitCast extends BinaryExpression
       > SELECT _FUNC_(map('a', 1, 'b', 2));
        2
       > SELECT _FUNC_(NULL);
-       -1
+       NULL
   """)
 case class Size(child: Expression) extends UnaryExpression with ExpectsInputTypes {
 

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

@@ -1865,7 +1865,7 @@ object SQLConf {
     .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.")
     .booleanConf
-    .createWithDefault(true)
+    .createWithDefault(false)
 
   val LEGACY_REPLACE_DATABRICKS_SPARK_AVRO_ENABLED =
     buildConf("spark.sql.legacy.replaceDatabricksSparkAvro.enabled")