There's even one more scenario! Suppose both the dealer and my partner are void in diamonds. The dealer tries to be clever and prevent an over-trump by playing a bower to trump my trick. Well, my partner cannot take the trick anymore - the dealer foiled their chances! You may think we are lost. But in fact, my partner has two trump cards: the queen and the ace. If the dealer leads the next hand with another bower, then my partner will wisely follow with the queen, and hold onto the ace. Then they will still spoil the loner.
As you can see, there are sometimes ways to stop a loner. The next time your opponent goes alone, and you are first to lead, think about what you can possibly do to help your partner take a trick. That is one way to defend against loners.