[DAR-1562][External] Allow pulling of unpopulated required property annotations

[JBWilkie]

Problem

When pulling annotations with unpopulated required properties, the frame_index field will be set to null. This caused a Pydantic ValidationError because it needs to be type int

Solution

It's required that darwin-py can pull unpopulated required properties (but not import them), so allow the SelectedProperty frame_index attribute to be None

Changelog

Fixed bug preventing darwin-py from being able to pull annotations with unpopulated required properties

[linear[bot]]

DAR-1562 BUG: darwin-py cannot pull exports if any required property in the export is not populated