CustomSelectControl: Provide fallback describedBy when there's no selected item

[Mamaduka]

Description

Currently if CustomSelectControl value is undefined component will trigger an error. The component tries to generate the describedBy value using the selected option's name. We should provide fallback value when there's no selection.

A similar change should be made for the FontAppearanceControl component as well.

Step-by-step reproduction instructions

1. Install and activate Quadrat theme.
2. Go the the "Site Editor."
3. The editor crashes and displays an error message.

Screenshots, screen recording, code snippet