Persistent trash (#21217)

# Summary

This PR adds trash to the persistence system.

- Trash becomes persistent the moment it is dropped unless it is dropped
in a maint or the disposal room.
- It looses persistence when it lands in a maint/disposal room or gets
picked up again.
- Persistent trash can be found up to three days.

Those are the base rules, some trash types have additional rules.

List of trash to be implemented:

- [x] Basic trash (snacks and such)
  - Cigarette buts
  - spitwad
  - Burned matches
- [x] Broken bottles
- [x] Torn inflatable (-doors)

> Littering is a i120 violation and can be punished with 3-7 minutes of
prison time or with a 40-150 credit fine.
> Stop littering and keep the ship clean.

## Follow-up tasks

- [x] Update GitHub Wiki: Add section on how to add persistent trash
types.
- [x] Update GitHub Wiki: Make persistence_get_content nullable/empty.

code/controllers/subsystems/persistence.dm

@@ -120,9 +120,11 @@ SUBSYSTEM_DEF(persistence)
  * RETURN: JSON formatted content of track or null if an exception occured.
  */
 /datum/controller/subsystem/persistence/proc/track_get_content(var/obj/track)
-	var/result = null
+	var/result = json_encode(list())
 	try
-		result = json_encode(track.persistence_get_content())
+		var/list/content = track.persistence_get_content()
+		if(content && content.len)
+			result = json_encode(content)
 	catch(var/exception/e)
 		log_subsystem_persistence("Track: Failed to get/encode track content: [e]")
 	return result
@@ -202,10 +204,6 @@ SUBSYSTEM_DEF(persistence)
 	if(!SSdbcore.Connect())
 		log_subsystem_persistence("SQL ERROR during persistence database_add_entry. Failed to connect.")
 	else
-		var/content = track_get_content(track)
-		if (!content)
-			return
-
 		var/turf/T = get_turf(track)
 		if(!T || !is_station_level(T.z)) // The persistence system only supports objects from the main map levels for multiple reasons, e.g. Z level value, mapping support
 			return
@@ -217,7 +215,7 @@ SUBSYSTEM_DEF(persistence)
 				"author_ckey" = track.persistence_author_ckey,
 				"type" = "[track.type]",
 				"expire_in_days" = track.persistance_expiration_time_days,
-				"content" = content,
+				"content" = track_get_content(track),
 				"x" = T.x,
 				"y" = T.y,
 				"z" = T.z
@@ -236,10 +234,6 @@ SUBSYSTEM_DEF(persistence)
 	if(!SSdbcore.Connect())
 		log_subsystem_persistence("SQL ERROR during persistence database_update_entry. Failed to connect.")
 	else
-		var/content = track_get_content(track)
-		if (!content)
-			return
-
 		var/turf/T = get_turf(track)
 		if(!T || !is_station_level(T.z)) // The persistence system only supports objects from the main map levels for multiple reasons, e.g. Z level value, mapping support
 			return
@@ -249,7 +243,7 @@ SUBSYSTEM_DEF(persistence)
 			list(
 				"author_ckey" = track.persistence_author_ckey,
 				"expire_in_days" = track.persistance_expiration_time_days,
-				"content" = content,
+				"content" = track_get_content(track),
 				"x" = T.x,
 				"y" = T.y,
 				"z" = T.z,