Inserting data into a Postgres table if data is not null

I'm using Postgres's Common Table Expressions, and would - if possible - to insert data into a table if the values in the data is not null:

WITH prod1 AS (
   INSERT INTO p1 ... RETURNING id), 
     prod2 AS (
   INSERT INTO p2 (fk,foo,bar) VALUES (prod1.id,@Foo,@Bar) 
     RETURNING id)

If the values @Foo and @Bar equate to null, is it possible for postgres to skip the insertion of data into p2 otherwise it'll create a blank reference? Or, do I have to manually check and insert as required via code?