-- https://docs.oracle.com/en/database/oracle/oracle-database/23/lnpls/plsql-language-fundamentals.html
DECLARE
  counter INTEGER;  -- initial value is NULL by default
BEGIN
  counter := counter + 1;  -- NULL + 1 is still NULL

  IF counter IS NULL THEN
    DBMS_OUTPUT.PUT_LINE('counter is NULL.');
  END IF;
END;
/