Just get the same ammonia used for fishless cycling and and then use the calculator on the site to add enough ammonia to produce 2 ppm in a tank 85% or so of the volume of yours (this allows for the glass itself and all the decor etc.). Add this amount every 2 - 3 days until you are ready to restock. This should keep the bacteria in tip top shape.
But, as noted above, the bacteria don't just roll over and die if their nutrients are not present. They will go into a dormant state. While dormant they stop reproducing and will eventually begin to die off slowly. But some manage to survive for a decent amount of time and will revive when the nutrients they need are once again available. Think of it as "bottled" bacteria which can survive in a bottle for 6 months in good shape, 12 if kept in the fridge. And even then they are not dead, they are just not helpful right out of the bottle or for more time thereafter than one wants when using such products. They would still be better than nothing at all at this stage.
So if your tank will only be empty for a week or two, you are pretty safe just keeping things wet. If you are looking at longer times or you want to insure your ability to add a full fish load when ready, then I suggest doing the ammonia dosing. If you are feeding the bacteria this way, before adding the new fish do a big water change and then test to be sure you have 0/0/low nitrate.
