Retain sign text when editing is aborted by <esc>

mods/default/nodes.lua

@@ -723,7 +723,7 @@ minetest.register_node("default:sign_wall", {
 			return
 		end
 		local meta = minetest.get_meta(pos)
-		fields.text = fields.text or ""
+		if not fields.text then return end
 		minetest.log("action", (sender:get_player_name() or "").." wrote \""..fields.text..
 				"\" to sign at "..minetest.pos_to_string(pos))
 		meta:set_string("text", fields.text)