Iterator improvements

[AThousandShips]

• Nullability safety for increment/decrement
• Style improvements

These are minor fixes and mostly style but also ensures iteration doesn't throw segfault when iterating past the end, this matches how it is done in HashMap which makes sense to apply everywhere IMO

Can limit it to just null fixes if desired 🙂

[RandomShaper]

Looks good. I'd only suggest using likely().