I wouldn't worry about it one way or the other. If you are super
worried, remove the automatic bleeder on the bottom of the tank
and install a ball valve or similar. Pressure the tank until the
regulator on the compressor is happy. Close off the compressor.
Open that bottom ball valve and blow down. If you are worried
about mold, mildew, unknown bugs, etc., make up some bleach
water - about 5 water to 1 bleach, empty the tank, pour in a
gallon of the mixture, shake vigorously (yes, I'm kidding), and
pressurize and blow down.
I can't imagine anything in the tank being unsafe. You will be
compressing your own "dirty air", I doubt that the steel tank is
holding much in the way of residual nasties.
Keep the whole world singing . . .
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