You don't want the water in your water heater is heated while the cold water is shut off. Your natural gas water heater should have a "vacation mode". You can just turn it to "vacation mode". Your water heater is not completely off when it is in "vacation mode". The pilot light in your water heater is still on. When you come back from your vacation, you can just turn it back to normal mode without a need of manually re-lighting it. Don't worry. It is very safe. If you still worry, you can just turn temperture down a low level.
Best regards,
Alex
