Follow-up to request #41814 Opening card in backlog might lead to a fatal error

After the fix for request #41814, I noticed that all of the cards in our taskboard have their little badge for the remaining effort. This badge is optional, and it only appears when the remaining effort is not empty (not null technically). Usually, it only appears for cards where we had entered a remaining effort, or when cards are closed, because the workflow automatically sets the remaining effort to zero. I found it strange that the badge was showing up for every card, most of the time we see it only for closed cards.

Introduced by d239615add51210f6fa7523ae4c84421ebcfac45: because we always cast the value, we change null values to 0, causing the card to appear when there is no remaining effort set. It also affects the API.