I want to return a null value in a formula

Content (please ensure you mask any confidential information):

I have the following questions:

Process Definition>Quote Process>Documents>Transaction Line>Attributes
Custom Attribute for CurrencyType
Default Tab>Function>Define Function

In the above formula, there may be cases where you want to return a null value at the end.
However, 「return null」 is an error.
At the same time, if you try to assign a null value to a variable (**** = null), an error will occur.

Looking at the error message, it seems that the null statement is being recognized as a variable, and the null statement itself is invalid.
However, since the isnull statement seems to be available, I wondered if there might be a way to return or assign a null value, so I asked.