How can I enforce a constraint only if a column is not null in Postgresql?

I would like a solution to enforce a constraint only if a column is not null. I can't seem to find a way to do this in the documentation.

create table mytable(
  table_identifier_a INTEGER,
  table_identifier_b INTEGER,
  table_value1,...)

Do to the nature of the data, I will have identifier b and a value when the table is created. After we receive additional data, I will be able to populate identifier a. At this point I would like to ensure a unique key of (identifier_a, value1) but only if identifier_a exists.

Hopefully that makes sense, Any one have any ideas?