Index on integer field in PostgreSQL

I don't think you will feel the difference in speed between NULL and some value. And, by the way, you can easily check for this simple test.

Asked the same question.
Generate a little data and verify whether the index of the query WHERE IS NULL ( I think 80% NULL will be fine)
If the plan shows the use of the index — so the index contains links to ALL of the NULL values, and, accordingly, there is no difference.
It would be interesting to create a sometimes compact codes for "sparse" values, where the references to NULL values are not stored, but usually in this case make the relationship one-to-one with another table.