[SPARK-16274][SQL] Implement xpath_boolean

## What changes were proposed in this pull request?
This patch implements xpath_boolean expression for Spark SQL, a xpath function that returns true or false. The implementation is modelled after Hive's xpath_boolean, except that how the expression handles null inputs. Hive throws a NullPointerException at runtime if either of the input is null. This implementation returns null if either of the input is null.

## How was this patch tested?
Created two new test suites. One for unit tests covering the expression, and the other for end-to-end test in SQL.

Author: petermaxlee <petermaxlee@gmail.com>

Closes #13964 from petermaxlee/SPARK-16274.