Return nil on empty get_area() (#4508)

doc/lua_api.txt

@@ -2886,6 +2886,7 @@ chosen for you.
 #### Methods
 * `get_area(id, include_borders, include_data)`: returns the area with the id `id`.
   (optional) Boolean values `include_borders` and `include_data` control what's copied.
+  Returns nil if specified area id does not exist.
 * `get_areas_for_pos(pos, include_borders, include_data)`: returns all areas that contain
   the position `pos`. (optional) Boolean values `include_borders` and `include_data` control
   what's copied.

src/script/lua_api/l_areastore.cpp

@@ -111,6 +111,9 @@ int LuaAreaStore::l_get_area(lua_State *L)
 	const Area *res;
 
 	res = ast->getArea(id);
+	if (!res)
+		return 0;
+
 	push_area(L, res, include_borders, include_data);
 
 	return 1;