[videodb] Don't remove TV shows with no episodes.

That's not configurable and this shouldn't be the default behavior.
[settings] Promote hideemptyseries to GUI setting.

This promotes videolibrary.hideemptyseries from an advanced setting to a GUI
one.  This is enabled by default.
[jsonrpc] GetTVShows: always return empty tv shows regardless of user setting
jsonrpc: bump to 6.21.2 after cc84c39bca83c2f5ff48e8e0b9fe427fae91d1a9.

JSON-RPC API version is incremented so that clients know that results before
6.21.2 may be unreliable.

(cherry picked from commit fc468a7ba4f07e0566193f90d7f9955c7ac5e7e1)