Fix type specialization and inference for empty lists.

This fixes the typeSpecialization test in test_types.py. The type of an empty list is written as *_ to indicate that the element type is undefined. In pylabrad, this is represented by LRList(LRNone()), since LRNone renders as '_'. However, we need to consider such an empty list as being a subtype of any other list type so that specialization will work properly.