check: _apply_opening_balance_correction helper method

[boscorat]

Couple of slight concerns we need to check for with _apply_opening_balance_correction helper method in the statements class in the statement.py.

1. What happens if there are no transactions on a statement - should this be bypassed? The opening balance will be correct in this scenario so we should probably skip.

2. We fire off a message if we've updated the opening balance with the calculated balance, but we update whether the update results in a new value or not. We should look at only updating when there's a change, or always updating but only triggering the message in the event of a value change.