[BE] refactor: Ticket의 school not null 제약조건 제거 (#465)

* refactor: Ticket의 school not null 제약조건 제거

* refactor: Ticket의 school not null 제약조건 제거

* refactor: Festival의 school not null 제약조건 제거

backend/src/main/java/com/festago/festival/domain/Festival.java

@@ -39,7 +39,6 @@ public class Festival extends BaseTimeEntity {
     @Size(max = 255)
     private String thumbnail;
 
-    @NotNull
     @ManyToOne(fetch = FetchType.LAZY)
     private School school;
 

backend/src/main/java/com/festago/ticket/domain/Ticket.java

@@ -39,7 +39,6 @@ public class Ticket extends BaseTimeEntity {
     @ManyToOne(fetch = FetchType.LAZY)
     private Stage stage;
 
-    @NotNull
     private Long schoolId;
 
     @NotNull

backend/src/main/resources/db/migration/V4__add_school_fk.sql

@@ -1,13 +1,13 @@
 alter table ticket
-    add column school_id bigint not null;
+    add column school_id bigint;
 
 alter table ticket
     add constraint fk_ticket__school
         foreign key (school_id)
             references school (id);
 
 alter table festival
-    add column school_id bigint not null;
+    add column school_id bigint;
 
 alter table festival
     add constraint fk_festival__school