In SQL Server you can use ISNULL function to replace NULL values. In PostgreSQL you can use COALESCE function.
SQL Server:
-- Replace NULL with N/A string SELECT ISNULL(NULL, 'N/A'); # N/A
PostgreSQL:
-- Replace NULL with N/A string SELECT COALESCE(NULL, 'N/A'); # N/A
For more information, see SQL Server to PostgreSQL Migration.