Not to start a fight with you but twice now, on the way back from a trek into the black, in an unarmed Dolphin, no cargo, no missions, no bounty, I have faced interdiction. Escaped it easily enough but it does appear to be random if rare.
Yes, it's unusual, but not out of the question to be interdicted with no missions, cargo or bounties. If it's a pirate, it will scan you and then leave complaining that it's children will go hungry... It's also possible to be interdicted by system authority ships patrolling in systems with war / civil war state. Don't know if that still happens, they would interdict, tell you to submit for a scan and then... Do nothing. So that particular scenario may have been dumped.
In my experience / opinion, interdictions have become much less of a thing as the game had gone on. I can do a dozen Elite ranked delivery missions and not get one incoming enemy alert, occasionally a random pirate, and if I do get an incoming enemy alert it's often as I'm docking... Haven't had a system authority interdiction in as long as I can remember.
I guess that for newer players of the game it can be a bit of a jolt - especially if they take on activities much higher ranked than their own rank and abilities or experience, and I suspect for those players four high ranking Anacondas coming after their Elite delivery cargo can seem like an endless cycle of interdictions. Once you get used to them, understand what might trigger them and have the ability to evade (very easy as time goes on) or submit and shoot them, also usually pretty easy depending on your ship and combat experience, then honestly the lack of them can become quite irritating.
This stuff should never have been done as random as it is for missions, it removes player agency, having to think about a mission you are accepting, consider whether you are equipped to deal with it. Missions should either come with a combat challenge that it clearly stated, or should be a milk run, with perhaps a random pirate thrown in on occasion, letting players choose how much challenge they want, and have the reward reflect that challenge. One can dream...
