No compressed air at all, travelers must remove the valve too. They will not take a cylinder with a closed valve on it. They think it will be full of a bio-weapon, and divers will unleash it upon unsuspecting holiday travelers.
Airlines are quite dry environments, it is when you release the air to empty the tank too quickly that you can get condensation. Likewise, if the tank is cold, and you take it out empty with no valve into a warm, humid environment, you could theoretically cause condensation to occur.