This depends entirely upon which ResMed CPAP you have. Though using un-humidified air can make your airways and nasal passages dry, cracked, or uncomfortable, that is not the biggest concern with not adding water to the humidifying chamber. Whether your ResMed CPAP can turn off the heated water function is actually the primary question. If your humidifier does not allow you to turn off the heat plate that warms the water, you will not want to utilize your CPAP without the liquid. If you were to use a CPAP with a heated humidifier without fluid in it, the excessive heat could damage the device.
However, some of the ResMed CPAPs can control the heat and humidity function up to and including the ability to not have the humidity at all. If your device is one that allows you to choose not to use humid air, then you should be ok using the device without water. Please remember, though, the moisture is necessary for any long-term sleep therapy. Without it, your sinuses can become very dry and cracked, which can leave you susceptible to sinus infections on top of being very uncomfortable.