There a cast to String but it does not work if the field is an integer. Actually it only works with string.
It should be better with something like :
String loginValue = (normalizedUserMap.get(loginColumnName) == null) ? "" : String.valueOf(normalizedUserMap.get(loginColumnName));
This is the whole source file corrected is attachment . This is wonderful
For your information I did
(normalizedUserMap.get(loginColumnName) == null) ? ""
because in the next line there a test isBlank and in order to take care about null value