make fields typed as lists optional

[miguelgrinberg]

I'm trying to type the examples, and another common issue I'm seeing is related to lists, either those created with multi=True or by using a Nested field.

The issue is with the state of the required option of the field. When using the non-typed field creation option this isn't a problem. Both Text(multi=True) and Nested(SomeDoc) work well with their default setting of required=False.

When using the new type hint definitions, however, the default for a typed field is to be required, and to make a field not required you have to wrap the type with Optional. This is okay for a scalar field, but for a list it creates confusion, because in terms of Python typing it is expected that an empty list is a valid value for a required field, but a required DSL field considers an empty list the same as None and does not allow it.

With this change, I'm changing the field creation logic to make sure that any fields that are created via type hints have required=False even when Optional is not given in the type hint, so that they work in the expected way and allow the empty list as a valid value.

No behavior changes are made for fields that are created without type hints.

[miguelgrinberg]

Closing. I have included this change in #1859

[pquentin]

Thanks! LGTM.