I don't like having any NULLs in my data model as they are difficult to work with and not selectable. I normally replace NULLs with '<Unknown>' or something similar.
MAP...USING works well:
MAP Field1,Field2,Field3,etc USING Map_Null_Unknown;
Now all nulls listed in the MAP...USING statement will be replaced as they are loaded. However, nulls created by LEFT/OUTER joins will not be replaced, unless that field is subsequently loaded later in the script. If your joined fields with nulls are not loaded again (by some kind of resident load) then you will need to force it. This works well:
RIGHT JOIN (Table) LOAD DISTINCT * RESIDENT Table;
Hope this helps,