Sql add condition IF

I have sql like :

SELECT * FROM leads_notes WHERE content <> '' AND lead_id <> ''

I need add rule if type <> close_task then write user_change_task_status IS NULL

My result sql is:

SELECT * FROM leads_notes WHERE content <> '' AND lead_id <> ''  IF(task_type <> 'close_task', 'AND user_change_task_status IS NULL',)

But i get many errors. Cant understand how can i solve this. Please help, thanks!