[BUG] Null Values Returned for Book and Member When Borrowing a Book

### Is this a new issue?

- [x] I have searched "open" and "closed" issues, and this is not a duplicate.

### Bug Description

When borrowing a book via the POST request to http://localhost:8080/api/borrowings, the response contains null values for both the book and member details. The expected behavior is to return the complete details of the book and member, but instead, the following response is received:
```json
{
"borrowingId": 1,
"book": {
"bookId": 2,
"title": null,
"author": null,
"isbn": null,
"publisher": null,
"publishedYear": 0,
"genre": null,
"copiesAvailable": 0
},
"fine": null,
"member": {
"memberId": 2,
"name": null,
"email": null,
"password": null,
"role": null,
"membershipDate": null
},
"borrowDate": "2024-09-05T17:37:49.623+00:00",
"dueDate": "2024-09-20T17:37:49.623+00:00",
"returnDate": null
}
```

### Steps to Reproduce

1. Borrow a book by sending a POST request to http://localhost:8080/api/borrowings with valid book and member data.
2. Check the response body for book and member details.

### Expected Behavior
- The response should contain the correct book and member details, such as title, author, ISBN, and member name, email, etc.