Sandy,
you could find these records if you use sqlscript..
select *
from <table>
where
"Customer" is null or
ltrim(rtrim("Customer")) = ''; -- verify this is your correct record set ... you may need additional filters or to cover other scenarios
once you identify the records, then you can run an update statement
update c
set "Customer" = 'AB'
where
"Customer" is null or
ltrim(rtrim("Customer")) = '';