test(entry_service): add a failure case test for create method

This commit adds a unit test for an important "unhappy path" in the
EntryService's `create` method.

It specifically verifies that the method gracefully returns `None`
when provided with a `travel_diary_id` that does not exist in the
database, ensuring the initial guard clause works as expected.

tests/service/test_entry_service.py

@@ -53,3 +53,18 @@ def test_create_entry_successfully(populated_db_session):
 
     entry_in_db = session.query(Entry).filter_by(id=created_entry.id).one()
     assert entry_in_db.title == "Primeiro Dia na Praia"
+
+def test_create_entry_fails_when_diary_id_is_invalid(db_session):
+    session = db_session
+    service = EntryService(session)
+    invalid_id = 666
+
+    result = service.create(
+        travel_diary_id=invalid_id,
+        title="Título de Teste",
+        text="Texto de Teste",
+        date=datetime(2025, 7, 20),
+        photos=[]
+    )
+
+    assert result is None