You can turn on a solid state amp without a load connected to it. But, just to be safe, you should contact Pass for their recommendation. I would not hook up any speakers until the amp has been turned off and on several times and has been allowed to run for a while with no obvious signs of distress (the most obvious sign of trouble will be a strong odor of something burning, repeated blowing of fuses or tripping of any safety relay that cuts the amp off).
The concern with an amp like this sitting a very long time unused is the state of the electrolytic capacitors in the power supply. With tube gear, you can soft start them by attaching them to a variac and using that to slowly bring up the voltage which allows the capacitor to "reform." But, some solid state gear do not like under-voltage conditions so this is not recommended unless Pass Labs says that this can be done.