Postgresql Unique Index on two columns with null value

Question

lets consider simple table

addresses_tab
id (SERIAL), user_id (INTEGER) address (VARCHAR)

It is possible to generate unique index on two columns:

user_id, address 

but only in case if field address IS NOT NULL

So valid case is:

15, 'SOME STREET'
15, NULL
15, 'ANOTHER STREET'
15, NULL

but invalid is

15, 'SOME STREET'
15, 'SOME STREET'