Do not fail on missing balance_date

The balance date is optional in OFX format. So we fall-back to the default
value if it is missing.

issue9777
review293041002

account.py

@@ -57,7 +57,10 @@ class StatementImport(metaclass=PoolMeta):
         if not isinstance(ofx_account.statement, ofxparse.Statement):
             raise ImportStatementError(
                 gettext('account_statement_ofx.msg_import_no_statement'))
-        statement.date = ofx_account.statement.balance_date.date()
+        try:
+            statement.date = ofx_account.statement.balance_date.date()
+        except AttributeError:
+            pass
         total_amount = sum(
             t.amount for t in ofx_account.statement.transactions)
         statement.total_amount = total_amount