Change left join to join so null Prerequisites does not break

src/main/java/nl/tudelft/ewi/tam/repositories/UserRepository.java

@@ -136,7 +136,7 @@ public class UserRepository {
             + "LEFT JOIN User_Role UR ON U.netid = UR.netid "
             + "LEFT JOIN Role R ON UR.role_id = R.id "
             + "WHERE name = \"STUDENT\") U "
-            + "LEFT JOIN Prerequisites P ON U.netid = P.netid";
+            + "JOIN Prerequisites P ON U.netid = P.netid";
 
     /**
      * Retrieve a {@link User} instance with the given netid.