Can the Stone Spire use Quake if there are no units to attack?

No. The Stone Spire has to be attacking in order for this ability to trigger before its attack. Quake reads: if this spire is on a mountain hex, before its attack during the spires fire step, you may remove the bottom upgrade from an opposing spire in range, remove 1 health from a landmark in range, or remove a facedown landmark in range from play. The facedown landmark is not considered defeated.