Set *_font_fallbacks default to None

[JunkuiZhang]

In the current default.json, *_font_fallbacks=[], which results in the fallbacks value in the Font struct always being Some(...).

This PR introduces the following improvements:

1. Changed *_font_fallbacks = [] to *_font_fallbacks = null in default.json.
2. Enhanced the macOS and Windows implementations.

Release Notes:

• N/A

[mikayla-maki]

Thanks!