SQL & PL/SQL :: Optimizing Function Returning Integer?

I have the following function that I am using as a template for any function that executes a select statement and return a single value as an output.

The function is working but I wanted to take an expert opinion if it can be optimized.

CREATE OR REPLACE FUNCTION AFESD.F_AGR_GET_AGREEMENT_SERIAL
(I_NUMBER0 IN NUMBER, S_SUB_NUMBER VARCHAR2 DEFAULT NULL, I_TYPE_ID NUMBER)

[Code]....

In addition I want to use the parameter S_SUB_NUMBER that can be NULL and add it to the select statement of the cursor, but I dont know how to do that in one statement.

CURSOR C_AGREEMENT
IS
SELECT AGREEMENT_SERIAL
FROM VW_AGR_AGREEMENT
WHERE NUMBER0 = I_NUMBER0
AND TYPE_ID = I_TYPE_ID
-->and sub_number is null;
-->and sumb_number = s_sub_number