I have had a lot of instances of the ai suiciding into "undefended" planets because they don't seem to properly assess the threat of what is there and don't send enough ships.
The main issue is Space Stations though and not just the Empress. They are all quite strong and as far as I can tell the ai doesn't even seem to realize they are there at all.
For example I had an Empress and Golan 2 and level 2 station on Kashyyyk with two corvettes and two marauders also. The Empire attacked me with just 5 MTC's and two SD's. You could probably even beat that attack without the Empress at all. The Empire has the strongest military score in this campaign and they don't have a ton of borders so I know they have more ships they could use.
In 2.2.0 I had the Yvetha attack an Oto Space Station and Visvia with 4 Thrustships, a Vic, and an Interdictor.
Those are just two examples but it happens consistently enough that You can just drop an Empress somewhere and move on with your fleet and they will never defeat it.
They also don't seem to count HVG or Ion cannon as part of their threat assessment either but I'd imagine that isn't easy to code for. I don't know how the ai decides on how many ships to attack with but whatever it is they are doing, they aren't respecting the stations well enough.