GSettings schemas: allow for zero items in schema
GSettings schemas: allow for zero items in schema

GVDB deals with empty lists by returning NULL for the list instead of a
zero-length (non-NULL) strv.  We can work around that in GSettingsSchema
by checking for the NULL case and treating it like a zero-length list.

https://bugzilla.gnome.org/show_bug.cgi?id=660147

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