[MRG] Cleaning for the fix lp:1313689:
* The _insert_lines and _update_line method in *_base_completion/statement.py calls the symbol_f method to prorperly initiate the default value (e.g. integer default null value should be Null not False).
* The mechanism that allow to have a null account_id in bank statement line has been move in *_base_completion, instead of *_base_import.