add unittest and comments

Author: zhichao-li

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

@@ -140,6 +140,12 @@ case class Cos(child: Expression) extends UnaryMathExpression(math.cos, "COS")
 
 case class Cosh(child: Expression) extends UnaryMathExpression(math.cosh, "COSH")
 
+/**
+ * Convert a num from one base to another
+ * @param numExpr the number to be converted
+ * @param fromBaseExpr from which base
+ * @param toBaseExpr to which base
+ */
 case class Conv(numExpr: Expression, fromBaseExpr: Expression, toBaseExpr: Expression)
   extends Expression with ImplicitCastInputTypes{
 
@@ -272,7 +278,7 @@ case class Conv(numExpr: Expression, fromBaseExpr: Expression, toBaseExpr: Expre
    * NB: This logic is borrowed from org.apache.hadoop.hive.ql.ud.UDFConv
    */
   private def conv(n: Array[Byte] , fromBase: Int, toBase: Int ): UTF8String = {
-    if (n == null || fromBase == null || toBase == null) {
+    if (n == null || fromBase == null || toBase == null || n.isEmpty) {
       return null
     }
 

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

@@ -103,8 +103,13 @@ class MathFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     checkEvaluation(Conv(Literal("3"), Literal(null), Literal(16)), null)
     checkEvaluation(
       Conv(Literal("1234"), Literal(10), Literal(37)), null)
+    checkEvaluation(
+      Conv(Literal(""), Literal(10), Literal(16)), null)
     checkEvaluation(
       Conv(Literal("9223372036854775807"), Literal(36), Literal(16)), "FFFFFFFFFFFFFFFF")
+    // If there is an invalid digit in the number, the longest valid prefix should be converted.
+    checkEvaluation(
+      Conv(Literal("11abc"), Literal(10), Literal(16)), "B")
   }
 
   test("e") {