fix: missing services dependencies inialization

inserted proper initialization to the dependencies from the service layer by their constructors

Author: vianavitor-dev

src/main/java/com/vianavitor/simplelibrarygame/service/AuthorService.java

@@ -18,7 +18,9 @@ public class AuthorService {
     private Author author;
 
     @Autowired
-    public AuthorService() {}
+    public AuthorService() {
+        this.author = new Author();
+    }
 
     public AuthorService(AuthorRepository repository, Author author) {
         this.repository = repository;

src/main/java/com/vianavitor/simplelibrarygame/service/ClassroomService.java

@@ -32,7 +32,9 @@ public class ClassroomService {
     private Classroom classroom;
 
     @Autowired
-    public ClassroomService() { }
+    public ClassroomService() {
+        this.classroom = new Classroom();
+    }
 
     public ClassroomService(
             ClassroomRepository repository, ProfessorRepository professorRepository,

src/main/java/com/vianavitor/simplelibrarygame/service/GroupOfBookService.java

@@ -28,7 +28,9 @@ public class GroupOfBookService {
     private GroupOfBook groupOfBook;
 
     @Autowired
-    public GroupOfBookService() {}
+    public GroupOfBookService() {
+        this.groupOfBook = new GroupOfBook();
+    }
 
     public GroupOfBookService(
             GroupOfBookRepository repository, BookRepository bookRepository,

src/main/java/com/vianavitor/simplelibrarygame/service/GroupService.java

@@ -20,6 +20,11 @@ public class GroupService {
 
     private Group group;
 
+    @Autowired
+    public GroupService() {
+        this.group = new Group();
+    }
+
     public Group create(String name, Long studentId) throws ResourceNotFoundException {
         Student owner = studentRepository.findById(studentId)
                 .orElseThrow(() -> new ResourceNotFoundException("not found student"));

src/main/java/com/vianavitor/simplelibrarygame/service/StudentService.java

@@ -37,7 +37,9 @@ public class StudentService implements ManageableUser<Student> {
     private StudentStats stats;
 
     @Autowired
-    public StudentService() {}
+    public StudentService() {
+        this.stats = new StudentStats();
+    }
 
     public StudentService(
             StudentRepository repository, StudentStatsRepository statsRepository,

src/main/java/com/vianavitor/simplelibrarygame/service/StudentStatsService.java

@@ -38,7 +38,10 @@ public class StudentStatsService {
     private BookReadHistory history;
 
     @Autowired
-    public StudentStatsService() {}
+    public StudentStatsService() {
+        this.stats = new StudentStats();
+        this.history = new BookReadHistory();
+    }
 
     public StudentStatsService(
             StudentStatsRepository repository, StudentRepository studentRepository,