MYSQL LEFT JOIN returns NULL on empty table?

Question

MySQL

How? Why? The tables are empty.

SELECT 
    st.*, MAX(sp.date) as last_date,
    SUM(case when sp.user_id != 3 then sp.status end) as unread 
FROM support_ticket st 
LEFT JOIN support_message sp ON sp.ticket_id = st.id 
WHERE st.user_id = 3

Execution result

ROW COUNT (1)
+-----------------------------------------------------------------------------------------------------------------+
|   id    | user_id  |  title  | service | type  | express | status | open_date | close_date | last_date | unread |
+-----------------------------------------------------------------------------------------------------------------+
|   NULL  |  NULL    |   NULL  |   NULL  | NULL  |   NULL  | NULL   |   NULL    | NULL       | NULL      | NULL   |
+-----------------------------------------------------------------------------------------------------------------+

Answer 1

Returns an error, an error.
And when ONLY_FULL_GROUP_BY is off, it will return any nonsense.