[SPARK-31468][SQL] Null types should be implicitly casted to Decimal types

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

This PR intends to fix a bug that occurs when comparing null types to decimal types in master/branch-3.0;
```
scala> Seq(BigDecimal(10)).toDF("v1").selectExpr("v1 = NULL").explain(true)
org.apache.spark.sql.AnalysisException: cannot resolve '(`v1` = NULL)' due to data type mismatch: differing types in '(`v1` = NULL)' (decimal(38,18) and null).; line 1 pos 0;
'Project [(v1#5 = null) AS (v1 = NULL)#7]
+- Project [value#2 AS v1#5]
   +- LocalRelation [value#2]
...
```
The query above passed in v2.4.5.

### Why are the changes needed?

bugfix

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

No.

### How was this patch tested?

Added tests.

Closes apache#28241 from maropu/SPARK-31468.

Authored-by: Takeshi Yamamuro <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: maropu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercion.scala

@@ -842,15 +842,26 @@ object TypeCoercion {
    * Casts types according to the expected input types for [[Expression]]s.
    */
   object ImplicitTypeCasts extends TypeCoercionRule {
+
+    private def canHandleTypeCoercion(leftType: DataType, rightType: DataType): Boolean = {
+      (leftType, rightType) match {
+        case (_: DecimalType, NullType) => true
+        case (NullType, _: DecimalType) => true
+        case _ =>
+          // If DecimalType operands are involved except for the two cases above,
+          // DecimalPrecision will handle it.
+          !leftType.isInstanceOf[DecimalType] && !rightType.isInstanceOf[DecimalType] &&
+            leftType != rightType
+      }
+    }
+
     override protected def coerceTypes(
         plan: LogicalPlan): LogicalPlan = plan resolveExpressions {
       // Skip nodes who's children have not been resolved yet.
       case e if !e.childrenResolved => e
 
-      // If DecimalType operands are involved, DecimalPrecision will handle it
-      case b @ BinaryOperator(left, right) if !left.dataType.isInstanceOf[DecimalType] &&
-          !right.dataType.isInstanceOf[DecimalType] &&
-          left.dataType != right.dataType =>
+      case b @ BinaryOperator(left, right)
+          if canHandleTypeCoercion(left.dataType, right.dataType) =>
         findTightestCommonType(left.dataType, right.dataType).map { commonType =>
           if (b.inputType.acceptsType(commonType)) {
             // If the expression accepts the tightest common type, cast to that.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercionSuite.scala

@@ -1541,6 +1541,24 @@ class TypeCoercionSuite extends AnalysisTest {
       Multiply(CaseWhen(Seq((EqualTo(1, 2), Cast(1, DecimalType(34, 24)))),
         Cast(100, DecimalType(34, 24))), Cast(1, IntegerType)))
   }
+
+  test("SPARK-31468: null types should be casted to decimal types in ImplicitTypeCasts") {
+    Seq(AnyTypeBinaryOperator(_, _), NumericTypeBinaryOperator(_, _)).foreach { binaryOp =>
+      // binaryOp(decimal, null) case
+      ruleTest(TypeCoercion.ImplicitTypeCasts,
+        binaryOp(Literal.create(null, DecimalType.SYSTEM_DEFAULT),
+          Literal.create(null, NullType)),
+        binaryOp(Literal.create(null, DecimalType.SYSTEM_DEFAULT),
+          Cast(Literal.create(null, NullType), DecimalType.SYSTEM_DEFAULT)))
+
+      // binaryOp(null, decimal) case
+      ruleTest(TypeCoercion.ImplicitTypeCasts,
+        binaryOp(Literal.create(null, NullType),
+          Literal.create(null, DecimalType.SYSTEM_DEFAULT)),
+        binaryOp(Cast(Literal.create(null, NullType), DecimalType.SYSTEM_DEFAULT),
+          Literal.create(null, DecimalType.SYSTEM_DEFAULT)))
+    }
+  }
 }