?? nothing wrong with nitrates being at zero if you don't have a planted tank. nitrates just irritate the fish and encourage algae (and your plants if you had some, and some cyano bacteria). it is odd that a barely-noticeable film of plants could have absorbed all the nitrates into a tiny mass of plant material.
i doubt that it's your test kit unless you left the bottles unsealed and warm. even if the test kit is old, it was working apparently OK before, and it shouldn't suddenly get old