default IsNull<T, R> isNull() {
        return new IsNullImpl<>(this);
    }
Returns a {@code Predicate} that is {@code true} for any values that
would make this expression return {@code null} and {@code false} for any
values that would make this expression return a result.

@return  a predicate that tests if this expression would return
         {@code null}
runtime-parent/runtime-compute/src/main/java/com/speedment/runtime/compute/trait/ToNullable.java