Fix NullPointerException for students that did not have their studno stored. (#13)

Also fix a method name typo.

src/main/java/nl/tudelft/ewi/tam/beans/User.java

@@ -212,6 +212,12 @@ public class User {
      * @return value of the {@link Integer} instance representing the student number of the user
      */
     public Integer getStudentNumber() {
+        if (studentNumber == null) {
+            // Because some students don't have a student_number in the database.
+            // Ref: https://gitlab.ewi.tudelft.nl/eip/tam/tam/issues/13.
+            studentNumber = 0;
+        }
+
         return studentNumber;
     }