[SPARK-42384][SQL] Check for null input in generated code for mask function

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

When generating code for the mask function, call `ctx.nullSafeExec` to produce null safe code.

This change assumes that the mask function returns null only when the input is null (which appears to be the case, from reading the code of `Mask.transformInput`).

### Why are the changes needed?

The following query fails with a `NullPointerException`:
```
create or replace temp view v1 as
select * from values
(null),
('AbCD123-$#')
as data(col1);

cache table v1;

select mask(col1) from v1;

23/02/07 16:36:06 ERROR Executor: Exception in task 0.0 in stage 3.0 (TID 3)
java.lang.NullPointerException
	at org.apache.spark.sql.catalyst.expressions.codegen.UnsafeWriter.write(UnsafeWriter.java:110)
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIteratorForCodegenStage1.processNext(Unknown Source)
	at org.apache.spark.sql.execution.BufferedRowIterator.hasNext(BufferedRowIterator.java:43)
	at org.apache.spark.sql.execution.WholeStageCodegenExec$$anon$1.hasNext(WholeStageCodegenExec.scala:760)
```
The generated code calls `UnsafeWriter.write(0, value_0)` regardless of whether `Mask.transformInput` returns null or not. The `UnsafeWriter.write` method for `UTF8String` does not expect a null pointer.

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

No.

### How was this patch tested?

New unit tests.

Closes #39945 from bersprockets/mask_npe_issue.

Authored-by: Bruce Robbins <bersprockets@gmail.com>
Signed-off-by: Gengliang Wang <gengliang@apache.org>
(cherry picked from commit 7ff8ba2)
Signed-off-by: Gengliang Wang <gengliang@apache.org>

Author: bersprockets

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

@@ -223,16 +223,32 @@ case class Mask(
     val fifthGen = children(4).genCode(ctx)
     val resultCode =
       f(firstGen.value, secondGen.value, thirdGen.value, fourthGen.value, fifthGen.value)
-    ev.copy(
-      code = code"""
+    if (nullable) {
+      // this function is somewhat like a `UnaryExpression`, in that only the first child
+      // determines whether the result is null
+      val nullSafeEval = ctx.nullSafeExec(children(0).nullable, firstGen.isNull)(resultCode)
+      ev.copy(code = code"""
+        ${firstGen.code}
+        ${secondGen.code}
+        ${thirdGen.code}
+        ${fourthGen.code}
+        ${fifthGen.code}
+        boolean ${ev.isNull} = ${firstGen.isNull};
+        ${CodeGenerator.javaType(dataType)} ${ev.value} = ${CodeGenerator.defaultValue(dataType)};
+        $nullSafeEval
+      """)
+    } else {
+      ev.copy(
+        code = code"""
         ${firstGen.code}
         ${secondGen.code}
         ${thirdGen.code}
         ${fourthGen.code}
         ${fifthGen.code}
         ${CodeGenerator.javaType(dataType)} ${ev.value} = ${CodeGenerator.defaultValue(dataType)};
         $resultCode""",
-      isNull = FalseLiteral)
+        isNull = FalseLiteral)
+    }
   }
 
   /**

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

@@ -440,6 +440,12 @@ class StringExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     }
   }
 
+  test("SPARK-42384: Mask with null input") {
+    val NULL_LITERAL = Literal(null, StringType)
+    checkEvaluation(
+      new Mask(NULL_LITERAL, Literal('Q'), Literal('q'), Literal('d')), null)
+  }
+
   test("string for ascii") {
     val a = $"a".long.at(0)
     checkEvaluation(Chr(Literal(48L)), "0", create_row("abdef"))

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

@@ -702,4 +702,16 @@ class StringFunctionsSuite extends QueryTest with SharedSparkSession {
       )
     )
   }
+
+  test("SPARK-42384: mask with null input") {
+    val df = Seq(
+      ("AbCD123-@$#"),
+      (null)
+    ).toDF("a")
+
+    checkAnswer(
+      df.selectExpr("mask(a,'Q','q','d','o')"),
+      Row("QqQQdddoooo") :: Row(null) :: Nil
+    )
+  }
 }