Allow interior `SchemaField` and `EmbeddedDataField` instances to take `undefined` or `null` if they can be initialized

[JPMeehan]

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• [ ] The darkness field in foundry.documents.BaseAmbientSound is a SchemaField that is not required and can be safely initialized from a value of an explicit undefined or null, but currently errors in the tests
• [ ] The texture field of foundry.documents.BaseNote is an EmbeddedDataField that is not required and can be safely initialized from a value of an explicit undefined or null, but currently errors in the tests

[LukeAbby]

This is a part of the broader issue that the check for if field.initial can be assigned to the field's assignment type is incomplete.