Typically, the stone is ruler straight, and the cut on the bottom of the backsplash is too, so, unless it is tipped, it is nearly a perfect connection in the first place. You won't get any into the joint, but you could put a bead at the joint. That often ends up looking funky, so, since it isn't something like a shower, I'd probably just leave it. You could try a bead of super glue, it is viscous enough where it might wick into the joint. It could stain the stone, so you'd want to try it first somewhere inconspicuous. I don't think you need to worry, as if you got enough water on there, it would likely just flow out into the room or down the sink. Just wipe up any large spills when they happen, and don't worry about it. Note, some stones can absorb moisture, and if yours can and it didn't have a sealer applied, you may want to do that. Some stone is so dense, it won't absorb a sealer, but some will suck it right up, and when wetted, change color. A sealer will help prevent things that can stain from penetrating, and will slow moisture color changes.