When a multi-valued ConfigurationSetting has no values set, the default is incorrectly used

[leonardr]

Consider a multi-select box such as the one for facets_enabled_available setting.

I had a library in which none of these boxes were checked. This caused problems in other parts of the system, but the value for the setting was correctly stored as "[]". However, in the admin interface, every box was checked, making it difficult to see what the actual problem was.

This is most likely a problem in the circulation manager, rather than the web interface, but it could be in the web interface instead. It's probably due to an if not value instead of if value is not None.