If you have access to change the destination table, just add a default to the column.
CREATE TABLE 1cust_det (
cust_id NOT NULL,
cust_name NOT NULL,
address NOT NULL DEFAULT 'DEFAULT_VALUE');
or if you can edit the existing destination table and it doesnt get drooped
ALTER TABLE 1cust_det
ALTER address SET DEFAULT 'DEFAULT_VALUE'
The easiest way if you don't have control of the destination table to add a default value to the address column is to use a case statement in the insert itself. In the example below you can also use a ISNULL evaluation, but you might want to search for empty strings as well. Please try to find a better way to insert instead of using a cursor.
INSERT dbo.1cust_det
(cust_id,cust_name,[address])
SELECT cust_id,cust_name,
CASE
WHEN [address] IS NULL THEN 'some default value'
ELSE [address]
END AS [address]
FROM cust