The inferred nullability of "is null" can at times be incorrect

In SQL, the nullability of "is null" should always be non-nullable, regardless of the input type.

Because of the short-cut here [1], the nullability is true. That shortcut is to avoid the following failure in WHERE clause:

WHERE booleanFunc(ANY type)

[1] https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/planner/sql/TypeInferenceUtils.java#L250