NULL in Oracle is represented with keyword NULL or ” (Two single quotes with without any space). However in PostgreSQL, NULL is identified only with the NULL keyword. If you provide ” (Two single quotes with without any space) then it is treated as empty string in PostgreSQL and has different behavior that that of NULL.
NULL in databases can cause problems if not handled properly while migrating queries as many functions, operators and other constructs behave differently in PostgreSQL. This is the reason why NULL handling is an important concept to look back at, while migrating Oracle database to PostgreSQL database.
Following are some articles which are good to start with for understanding NULL basics between Oracle and PostgreSQL database.