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

[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.