When profiling Dome Keeper, I found that in physics_process a HashMap
gets cleared a lot, which ends up calling the Variant destructor.
Calling Variant::clear() dominates this operation.
By not uselessly setting the Type to NIL on destruction we save about
50% of time. This is likely because if there is a simple type in the
Variant that doesn't need destructing, but now we write when we should
just drop the Variant altogether.
Since the value of Variant::type should be unobservable after
destruction this doesn't change any outward behavior.
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