SELECT  'Equal' FROM DUAL
 WHERE  NVL(NULL,'') = '';

Output

SELECT  'Equal' FROM DUAL
 WHERE  NVL(NULL,'~') = '~';

Output
Empty

While Passing Parameters to Procedure its should be

SELECT  'Equal' FROM DUAL
 WHERE  NVL(col_name, '~') = NVL(p_param, NVL(col_name, '~'));