Query multiple tables with foreign key filled or is null
This is what outer joins do. Use a LEFT (outer) JOIN:

select t1.id, t1.field1, t2.field2
from       table1 as t1
       left join 
           table2 as t2
       on  
           t1.id_table2 = t2.id ;


If you want the empty string (and not NULL) in the result when there is no match, replace t2.field2 in the select list with COALESCE(t2.field2, '').