True: only sentient beings can think about moral questions, and so moral questions don't arise in a world with no sentient (or better, sapient) beings.
Of course, in one sense of "arise," no questions arise unless there are creatures who can ponder the questions. Nonetheless, that doesn't make the way things are depend on the existence of thinking beings. There were electrons before we came on the scene, and there would be electrons even if neither we nor any creatures like us had existed.
That said, you're right: moral matters have an intrinsic connection with beings who can ponder them. There are no live moral issues in a lifeless world, nor even in one with sentient but no sapient creatures. Moral truths are truths about how certain kinds of creature should behave if there were any. But this is consistent with there being moral truths even if nothing in the world knows those truths and even if none of the relevant kinds of creatures exist. Thus, one might say (I would) that before any thinking beings existed it was true that if there were creatures such as us, it would be wrong for them to be cruel to one another.
Now the moral truth here takes a conditional form. Furthermore, it's not a truth that amounts to a constraint on the way the universe actually unfolds. But that doesn't get in the way of its being a truth.
Is it an objective truth? Well, it's not as though it could turn out to be false if thinking creatures came into being and believed that it was false. There's no contradiction or paradox in saying that these creatures would simply be wrong—rather like the way they'd be wrong if they thought that the square root of 2 is a rational number. That sure sounds like objectivity to me.
Of course one could doubt (some people do) that there are any moral truths even as things are—even given the existence beings like us who can contemplate them. But that strikes me as a rather different issue. The general point is that there could be truths that really are objective even though they are merely hypothetical unless certain kinds of things exist. That general point is the one I'm trying to apply here.