I just carbed a keg of RO water to start experimenting with mixing syrup on the fly and was wondering the same thing. I sanitized the keg as usual, boiled the water for a couple of minutes and added it to the keg hot to discourage any growth. Soda water is also acidic, since CO2 dissolved in water forms carbonic acid:
For a CO2 pressure typical of the one in soda drink bottles ( ~ 2.5 atm), we get a relatively acid medium (pH = 3.7) with a high concentration of dissolved CO2.
That should discourage most spoilage organisms and molds.