[banquepopulaire] Handle gocardless transactions that have hyperlink

Some Gocardless transactions do not have any href, leading to crashes
during iter_transactions when the set_gocardless_transaction_details()
method is called. This patch first checks if there is a link before
calling the method, thus preventing crashes during iter_history.

Closes: 5887@zendesk, 5746@zendesk, 5744@zendesk