settings: Include `null` in the type for optional settings (#20192)

This PR updates all settings that are defined as `Option`s to include
`null` in their type.

This prevents warnings from being displayed when `null` is used a
default value.

Closes https://github.com/zed-industries/zed/issues/18006.

Release Notes:

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