Problem with not required integer field after migration from D6

For testing, I did a new install drupal 6.24 English
I add cck module last version, I enable text and integer.
I add a new content type with an integer field, not required.
I add 3 contents (not important).
I migrate from D6 to D7, core, then cck

When I add a new content of that content type, with no value for integer, I get :
PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: '' for column 'field_entier_value' at row 1: INSERT INTO {field_data_field_entier} (entity_type, entity_id, revision_id, bundle, delta, language, field_entier_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] => 6 [:db_insert_placeholder_2] => 6 [:db_insert_placeholder_3] => nicole [: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\cmsd624en\modules\field\modules\field_sql_storage\field_sql_storage.module).

I ask here because since months there is no issue to that problem and for now, because of that, nearly all upgrades D6/D7 are stopped. For integer, it's easy to turn around problem by just giving a default value to zero, but for nodereference, it's not possible and problem is same as with integer.