Quarantine Tank
 

I always quarantine new fish when I receive them. I have 2 tanks that I use as quarantine tanks.

What do you do with a quarantine tank when you have no fish in it? Some articles say to break it down and empty it, but if you do this you have to start all over again with new tank syndrome each time you set it up.

After the quarantine period is over and you move the fish to their permanent tank, can you keep the quarantine tank cycled with no fish in it or do you have to keep fish or snails or something living in it for the Nitrogen cycle to continue?

You can keep it set up for awile as long as your filter is running.
I break mine down each time, I like to clean it all out just iin case anything bad left behind.
When I start it up I take the water from my main tank, and I always keep a sponge filter in my main tank, so it is ready and seeded with good bacteria. I also take a hand full of gravel or some filter media. This way it should not cycle.
Keep in mind it takes about 3-4 weeks for your sponge to get seeded with bacteria.

Yep, keep a sponge from the filter in your main tank so that you can start a QT instantly with a small tank and hang on filter.