In non-populated areas of space, as in those without stations, is it possible to have NPCs interdict / attack you? I keep seeing salvageable crafts appear as spawns within various systems.
The answer to the question in your title is clearly no! While there are salvageable craft from time to time, there are no NPC that could interdict or attack you. That being said, deep space is not the same as non-populated space without stations. NPCs will continue to show up outside of The Bubble (populated space) for some time.
However, as soon as you put a few long distance jumps (50 to 100 LY in total) between you and inhabited space you are safe from NPCs.