Migrated empty field malformed (field label shown)

Experienced with a text_list field:
* have an empty field in the source
* migrate it

Result: destination field seems empty, but the field label is shown.

Debugging shows that the field is malformed, containing a single NULL entry.

Array
(
    [und] => Array
        (
            [0] => Array
                (
                    [value] => 
                )

        )

)

while manual saving gives:

Array
(
)