settings: Allow `null` as a value for font fallback fields (#20186)

This PR updates the `buffer_font_fallbacks` and `ui_font_fallbacks`
settings to allow `null` as a value instead of showing a warning.

Related to https://github.com/zed-industries/zed/issues/18006.

Release Notes:

- Updated the settings schema to allow `null` as a value for
`buffer_font_fallbacks` and `ui_font_fallbacks` instead of showing a
warning.