Handle optional properties in bevy_settings

TOML has no `none` token, so field of type `Option` cannot be encoded properly in bevy_settings.

There are various workarounds, such as using an empty table to represent an "empty" value.

A different approach is to omit the key altogether, although this leads to an inconsistency with how other fields are handled, since it means that previous values would be preserved instead of being set to None.