Fix signs not updating on player leave

Avoid warning when deserializing missing players list

Empty string gets deserialized to nil (what we want).
nil gives a warning (and also gets deserialized to nil).

Make initial text consistent

Do not mutate given definitions

They may be shared.
signs_lib.standard_wood_groups for example IS shared, so it got mutated
and all signs that used it got treated as last_login_signs, overwriting
them all.

Add cc to LICENSE file too

as requested by the cdb reviewer

Link to cc text

🌅