## Why this should be merged
Fixes invariant broken by introduction of `types.StateAccountExtra`.
## How this works
`state.stateObject.empty()` now also requires that the underlying
account's `Extra` field carries the zero value if a type is registered,
or is itself nil.
## How this was tested
New unit test.
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Co-authored-by: Darioush Jalali <darioush.jalali@avalabs.org>
