The UI of admin_setting_configselect_autocomplete implies that the selected option can be removed by clicking 'x' next to it, resulting in selecting "none" of the options (unsetting) when settings are saved. This can be useful for letting user to choose an option to enable the feature when something has been selected, but removing selected element will indicate that the setting is disabled.

While there is a workaround by adding "empty" option deliberately, this does not look great in UI:

It would be nice to add a setter (e.g. set_emptyallowed) to make it behave more naturally, when none of options are selected (removed selected options), use null in config_write, effectively unsetting the setting.