[SPARK-2665] [SQL] Add EqualNS & Unit Tests

Hive Supports the operator "<=>", which returns same result with EQUAL(=) operator for non-null operands, but returns TRUE if both are NULL, FALSE if one of the them is NULL.
Can we spell this out? (i.e. `EqualsNullSafe`)  Since there is no conflict with Scala we can probably leave out the "To".