Add defensive check for learning purposes

Author: codesungrape

src/main/java/com/codesungrape/hmcts/BookAPI/service/BookService.java

@@ -39,6 +39,13 @@ public Book createBook(BookRequest request) {
                 // ID and created_at are auto-generated by JPA/DB
                 .build();
 
-        return bookRepository.save(newBook);
+        Book savedBook = bookRepository.save(newBook);
+
+        // Defensive check (even though it "shouldn't" happen)
+        if (savedBook == null) {
+            throw new IllegalStateException("Failed to save book - repository returned null");
+        }
+
+        return savedBook;
     }
 }