Treat 0 and NULL differently for output

Test Plan:
- Run this script and make sure 0 and NULL show up correctly in the results.
CREATE TABLE students (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT, grade_year INTEGER);
CREATE TABLE student_grades (id INTEGER, grade INTEGER, FOREIGN KEY (id) REFERENCES students(id));
INSERT INTO students VALUES (1, "Brian", 0);
INSERT INTO students VALUES (2, "kamil", NULL);
SELECT * FROM students;

Auditors: pamela