Concept of Polymorphism implemented with 'NULL'

In PL/SQL Syntax for basic IF-THEN-ELSE is as follows

IF(expression) THEN
Statement 1;
ELSE
local_variable:= Rvalue;
END IF;

Cosidering the above syntax to appreciate the concept of Polymorphism implemented through "NULL"

NULL AS STATEMENT
--------------------------
IF(condition is true) THEN
NULL; -- Null acts as a Statement
ELSE
execute this statement;
END IF;


NULL AS RVALUE
----------------------
IF(condition is true) THEN
local_variable := NULL; -- Null acts as a Rvalue
ELSE
execute this statement;
END IF;

The following example is only to appreciate the concept of Polymorphism

NULL AS AN EXPRESSION
-------------------------------
BEGIN
IF(NULL) THEN -- NULL acts as an expression
DBMS_OUTPUT.PUT_LINE('I am Null');
ELSE
DBMS_OUTPUT.PUT_LINE('I am not Null');
END IF;
END;

If the above autonomous procedure is executed it will always provide an o/p : I am not Null