Help with database change

I am doing some research into partitioning my postgres database. While
doing this, I am trying to take the opportunity to improve the over all
database.design. One of the things I am thinking of implementing would be
the use of nullif/coalesce. Our application tends to send a lot of strings
that only contain spaces. While I know in the long run, the application
needs to be fixed, I was thinking about using nullif and coalesce so the
database can just store nulls in the database and save the space of storing
"empty" strings as an interim fix.

My question is, how expensive is nullif and coalesce? Can I expect to see a
noticeable impact on performance if I do this?

Thanks,

Chris