[SPARK-16602][SQL] `Nvl` function should support numeric-string cases

## What changes were proposed in this pull request?

`Nvl` function should support numeric-straing cases like Hive/Spark1.6. Currently, `Nvl` finds the tightest common types among numeric types. This PR extends that to consider `String` type, too.

```scala
- TypeCoercion.findTightestCommonTypeOfTwo(left.dataType, right.dataType).map { dtype =>
+ TypeCoercion.findTightestCommonTypeToString(left.dataType, right.dataType).map { dtype =>
```

**Before**
```scala
scala> sql("select nvl('0', 1)").collect()
org.apache.spark.sql.AnalysisException: cannot resolve `nvl("0", 1)` due to data type mismatch:
input to function coalesce should all be the same type, but it's [string, int]; line 1 pos 7
```

**After**
```scala
scala> sql("select nvl('0', 1)").collect()
res0: Array[org.apache.spark.sql.Row] = Array([0])
```

## How was this patch tested?

Pass the Jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14251 from dongjoon-hyun/SPARK-16602.

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

@@ -100,7 +100,7 @@ object TypeCoercion {
   }
 
   /** Similar to [[findTightestCommonType]], but can promote all the way to StringType. */
-  private def findTightestCommonTypeToString(left: DataType, right: DataType): Option[DataType] = {
+  def findTightestCommonTypeToString(left: DataType, right: DataType): Option[DataType] = {
     findTightestCommonTypeOfTwo(left, right).orElse((left, right) match {
       case (StringType, t2: AtomicType) if t2 != BinaryType && t2 != BooleanType => Some(StringType)
       case (t1: AtomicType, StringType) if t1 != BinaryType && t1 != BooleanType => Some(StringType)

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

@@ -134,7 +134,7 @@ case class Nvl(left: Expression, right: Expression) extends RuntimeReplaceable {
 
   override def replaceForTypeCoercion(): Expression = {
     if (left.dataType != right.dataType) {
-      TypeCoercion.findTightestCommonTypeOfTwo(left.dataType, right.dataType).map { dtype =>
+      TypeCoercion.findTightestCommonTypeToString(left.dataType, right.dataType).map { dtype =>
         copy(left = Cast(left, dtype), right = Cast(right, dtype))
       }.getOrElse(this)
     } else {

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

@@ -77,6 +77,21 @@ class NullFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     }
   }
 
+  test("SPARK-16602 Nvl should support numeric-string cases") {
+    val intLit = Literal.create(1, IntegerType)
+    val doubleLit = Literal.create(2.2, DoubleType)
+    val stringLit = Literal.create("c", StringType)
+    val nullLit = Literal.create(null, NullType)
+
+    assert(Nvl(intLit, doubleLit).replaceForTypeCoercion().dataType == DoubleType)
+    assert(Nvl(intLit, stringLit).replaceForTypeCoercion().dataType == StringType)
+    assert(Nvl(stringLit, doubleLit).replaceForTypeCoercion().dataType == StringType)
+
+    assert(Nvl(nullLit, intLit).replaceForTypeCoercion().dataType == IntegerType)
+    assert(Nvl(doubleLit, nullLit).replaceForTypeCoercion().dataType == DoubleType)
+    assert(Nvl(nullLit, stringLit).replaceForTypeCoercion().dataType == StringType)
+  }
+
   test("AtLeastNNonNulls") {
     val mix = Seq(Literal("x"),
       Literal.create(null, StringType),