Fix section_occupancy: Return empty table

Fixes the functioning of the LuaATC function section_occupancy in
the presence of no trains.

Currently, if there is no train in the section,
advtrains.interlocking.db.get_ts will return a table with a nil entry.
When that nil value is passed to table.copy, Minetest throws out an error.
Instead of passing nil to table.copy, just make a new empty table.

advtrains_luaautomation/environment.lua

@@ -233,7 +233,7 @@ if advtrains.interlocking then
 		ts_id = tostring(ts_id)
 		local response = advtrains.interlocking.db.get_ts(ts_id)
 		if not response then return false end
-		return table.copy(response.trains)
+		return (response.trains and table.copy(response.trains)) or {}
 	end
 end