[SPARK-17884][SQL] To resolve Null pointer exception when casting from empty string to interval type.

## What changes were proposed in this pull request?
This change adds a check in castToInterval method of Cast expression , such that if converted value is null , then isNull variable should be set to true.

Earlier, the expression Cast(Literal(), CalendarIntervalType) was throwing NullPointerException because of the above mentioned reason.

## How was this patch tested?
Added test case in CastSuite.scala

jira entry for detail: https://issues.apache.org/jira/browse/SPARK-17884

Author: prigarg <prigarg@adobe.com>

Closes #15449 from priyankagargnitk/SPARK-17884.

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

@@ -657,7 +657,12 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
   private[this] def castToIntervalCode(from: DataType): CastFunction = from match {
     case StringType =>
       (c, evPrim, evNull) =>
-        s"$evPrim = CalendarInterval.fromString($c.toString());"
+        s"""$evPrim = CalendarInterval.fromString($c.toString());
+           if(${evPrim} == null) {
+             ${evNull} = true;
+           }
+         """.stripMargin
+
   }
 
   private[this] def decimalToTimestampCode(d: String): String =

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

@@ -767,6 +767,7 @@ class CastSuite extends SparkFunSuite with ExpressionEvalHelper {
   test("cast between string and interval") {
     import org.apache.spark.unsafe.types.CalendarInterval
 
+    checkEvaluation(Cast(Literal(""), CalendarIntervalType), null)
     checkEvaluation(Cast(Literal("interval -3 month 7 hours"), CalendarIntervalType),
       new CalendarInterval(-3, 7 * CalendarInterval.MICROS_PER_HOUR))
     checkEvaluation(Cast(Literal.create(