[BUG] Parallelize crashes when an exception row does not match normal case varlength schema

Tuplex crashes on `parallelize` when the normal case is a tuple with a var length field but an exception does not have a var length field

Example
```python3
c = Context()
c.parallelize([("a", 1), ("b", 2), ("c", 3), (None, 4)]) # Crashes because the normal case is (str, int) but an exception is (NoneType, int)
```