Hi
Yes, you can leave your watch on the charger without having power connected. The magnet will not harm the battery.
If power is instead left connected, Apple Watch cannot be overcharged and the battery still will not suffer any harm under normal use. Charging will stop automatically when it is fully charged (and restart as / when required due to ongoing battery usage). Follow Apple's safety and handling instructions and also note that:
- It is not possible to turn the watch off whilst it is on the charger and connected to power - this is normal.
- It is also normal for your watch, power adapter and charging cable to become warm when connected to power.
More information:
Check your battery and charge your Apple Watch - Apple Support
Keep Apple Watch within acceptable operating temperatures - Apple Support
https://www.apple.com/batteries/why-lithium-ion/