Don't fail on different nginx folder structure

[andreasnuesslein]

This commit should make the script a bit more robust in case somebody a) has deleted sites-available (because it's a ridiculous concept and it's totally fine to just put the relevant config files directly in sites-enabled) or b) has no files in sites-enabled at the moment.

I would even argue just deleting all of people's configs is not great either but I guess you have a "only do this on a fresh host" disclaimer somewhere.