item-04 `matita.office` Classes to `None` if defining com_object is `None`

There are operations which return None (null or nothing in VBA). To be more intuitive, the class instance should be set to None directly if the initiation COM object is already None.

cell1 = wks.cells(1,1)
cell2 = wks.cells(2,2)
rng = xl_app.intersect(cell1, cell2)

print(rng.com_object is None) # True
print(rng is None) # False, should be True instead