ListField `items_types` parameter should be required

Right now this parameter has a `None` default value, which means that the actual value will be an empty tuple.

This creates inconsistent behavior:
```python
class Foo(Base):
l = ListField() # Look ma, no `items_types`!

f = Foo()
f.l.append("a") # works

# But look at this:
f = Foo(l=["a"])
Traceback (most recent call last):
...
ValidationError: Cannot decide which type to choose from "".
```

Inconsistency: Validation during ``__init__`` is not the same as when using `append`.