MySQL self join return all rows

Per the example data below, I need a query that returns every row, where if the 'contingent_on' field is NULL, it is returned as NULL, but if it is not NULL it is returned with the 'ticket_name' corresponding to the 'primary_key' value.

I tried self join queries but could only get them to return the not NULL rows.

example table data:

primary_key | ticket_name | contingent_on

1 | site preparation | NULL

2 | tender process | NULL

3 | construction | 1

All rows should be returned, where the in the 'construction' row return, 'site preparation' is input in place of '1' in the 'contingent_on' field.