Config item to define "empty" text per placeholder

[Dantevg]

Some placeholders return arbitrary text when a player is not online, instead of null, the empty string or 0. Mostly, this is not a problem if store-placeholders-in-file is enabled. But some servers have old players that have no other scores than this dummy player-not-online score, and these players should not show up (they do now). A config item (probably something like placeholder-empty-values) that defines the "empty" value per placeholder will fix this.