Replacing Null Values not working

[#1]

Hello - this is a snippet from my code. I cannot get the nulls to populate the no payments. I have tried a number of techniques (isNull, len=0, combinations of resident tables etc)  but to no avail. Please can you help.

Many thanks, James.

NO_PAYMENT_MAP:
MAPPING LOAD
null(),'NO PAYMENTS'
AUTOGENERATE 1;
map "Last_Paid" using NO_PAYMENT_MAP; //not sure about quotation marks, single quotes, or nothing at all.
 
PAYMENTS:
LOAD Document_Type, 
     date(Posting_Date) as Posting_Date, 
     CA, 
     Amount as Amount_2
     
FROM

(qvd)
where Exists(CA)
;

MAX_DATE:
Load
 CA,
  date(max(Posting_Date)) as Last_Paid
  Resident PAYMENTS
 Group By CA
 ;

MAX_DATE_2:
NoConcatenate
Load
CA,
Last_Paid

//I also tried... if(isNull(Last_Paid), 'No Payment', Last_Paid) as Last_Paid
Resident MAX_DATE
;

DROP TABLE MAX_DATE;
 
MAX_DATE_FINAL:
NoConcatenate
Load *
Resident MAX_DATE_2;

DROP TABLE MAX_DATE_2;