++++++++++++++++++
Correct validation behaviors when fields are used in embedded serializers. Also correction to the
`list` and `dict` type checks for `None` values (15, 16, 18).
* Implement `initialize` and `field_from_native` to ensure proper validation in embedded
serializers.
* Give the fields distinct `validate` and `run_validators` implementations that don't call each
other.
* Don't apply the `list` and `dict` type checks for `None` values.