Ampscript Lookup function on field which is null

I am using this function:

%%=Lookup("ENT.SBQQ__QuoteLine__c_Salesforce","SBQQ__ProductName__c","sbqq__quote__c", "a0n5E000002KZHQQA4", "SBQQ__Required", @required)=%%

which works all the times (it returns the proper value) except when @required is null (it should return a string in such case as well).

I tried with @required = '' or @required = null but with no luck.

How can I solve it?

My goal is to get SBQQ__ProductName__c where sbqq__quote__c is a0n5E000002KZHQQA4 AND where SBQQ__Required is null.

thank you