I'm not very knowledgeable on this topic since I haven't done Modern History, but I don't think Stalin would have broken the Molotov–Ribbentrop Pact if Hitler hadn't first.
If we observe the character traits of each of those leaders, we can deduce that Stalin was more positively depicted than Hitler. Stalin was described using terms such as "bold", "wise" and "genius", whereas Hitler was described as "lacking education", "manipulative", "selfish" and "an angry hater". This could indicate that Stalin would have not breached the pact himself if Hitler hadn't because this pact could be of strategic importance to the Soviet Union.
Another factor I would consider is the fact that the Molotov–Ribbentrop Pact was signed in the Kremlin, Russia, which might not be seen as significant, but the fact that the Soviet Union would contribute to this pact by hosting meetings between Soviet politicians and their German counterparts within Soviet Union territory could potentially be an indicator of the seriousness of the Soviet Union to make this pact work, meaning Stalin would've probably not breached it because the Soviet Union was supportive of it.
I hope this helps!