The weakest part of any nipple will be any nipple threads that aren't engaged with the spoke threads. There, you'll have a hollow core. To prevent this, you need the spoke threads reaching at least the bottom of the nipple slot.
Take a straw, plug one end, fill it halfway with water, and freeze it standing upright, plug-side down. Then bend the straw. I'm willing to bet it will crease above the ice line, while the ice-filled section remains in tact.