How i prevent customer invoice no field data exist from updated when i transfer empty data to same field

Hi guys can any one help me in this task

I create field customer invoice no in Database B and Database A to give every group of sales invoice line unique number

this processes done IN Database B customer invoice no is sales invoice line table when this field is empty in same sales invoice no it remove value that found in database B

ex

Database A                                         Database B

Sales invoice line table                    Sales invoice line table

No                customer invoice no           No        customer invoice no

so-30                                                         so-30      300

When i transfer data from Database A to database B in table sales invoice line

it clear or remove 300 and become no value

How i put condition by code tell him

when import

if sales invoice line table .customer invoice no field has value

not updated

Are this possible by code please help me

thanks.