Yes, we have 0 evidence for a gun port -- but we also have 0 evidence against a gun port.
My point being that even when there is a gun port on eventual production aircraft we probably wouldn't be able to see it if the photos we get are the same quality as we have at present.
In other words, debating over whether current J-20 prototypes has a gun based off current photo evidence may be pointless and illogical, because photos may not useful for determining the existence of a gun on the aircraft regardless of whether it is a prototype or a production aircraft.
I repeat, if J-20 does have a gun, we probably would not know about it and we would be unable to ID it, given the quality of our current photos and also how unobtrusive a gun port can be on a stealth fighter.
So saying "we can't see a gun therefore the prototypes do not have a gun" is illogical.
yes Mr. Spock!