As we are obviously into pedantry on this thread, PENKO, the tank would not be "full" or "empty" - "charged" or "discharged" it may be - I would suggest it would always be "full" of something, albeit the pressure and density may change ...
JD
Ouch!
Let's keep it simple, I'm no scientist. If a newly pressurized scuba tank weighs more than one that has been used for a one hour dive then for sure any airplane being pressurized will weigh more than an unpressurized one in otherwhise equal circumstances.