fix error parsing field 'original_amount' when it is not being provided in

the mapping.

account_bank_statement_import_txt_xlsx/models/account_bank_statement_import_sheet_parser.py

@@ -220,7 +220,10 @@ class AccountBankStatementImportSheetParser(models.TransientModel):
             elif original_currency == currency:
                 original_amount = amount
 
-            original_amount = self._parse_decimal(original_amount, mapping)
+            if original_amount is not None:
+                original_amount = self._parse_decimal(original_amount, mapping)
+            else:
+                original_amount = 0.0
 
             line = {
                 'timestamp': timestamp,