Problem afterwards with migrated integer or nodereference field when empty

I still have problem with integer fields I've migrated.
Fields are not required. When I create a new node, if I leave the field empty I have that pdo exception

PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: '' for column 'field_altitude_bas_value' at row 1: INSERT INTO {field_data_field_altitude_bas} (entity_type, entity_id, revision_id, bundle, delta, language, field_altitude_bas_value) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6); Array ( [:db_insert_placeholder_0] => node [:db_insert_placeholder_1] => 2904 [:db_insert_placeholder_2] => 2904 [:db_insert_placeholder_3] => bisse [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => ) in field_sql_storage_field_storage_write() (line 448 of C:\Program Files (x86)\EasyPHP-5.3.8.1\www\cms\mdbMig50\modules\field\modules\field_sql_storage\field_sql_storage.module).

In fact, Drupal is trying to create a record in table field_data_field_xxx with value blank or something illegal.
Normally, for new field integer or noderef created in D7, when field stay empty or nothing is selected in select list, Drupal does not create a record.

This problem make it impossible to migrate from D6 to D7 if we have node reference fields that are not required.