As a kludge workaround, you could try changing that to include a default that is your Site ID, and then delete and re-add the device. E.g., something like
Make sure to get into the IDE by going to account.smartthings.com. After logging in, select “My Device Handlers” and then this handler. After editing the code, click “Save” and then “Publish → For Me”.
What happens if you go into the IDE, select Devices, click on your Solar Edge Device, scroll down to Preferences and click on Edit? Can you enter the full SiteID there?