Set `*_font_fallbacks` default to `None` (#16941)

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