luci: app_update use default_path when not set in config

luci-app-passwall/luasrc/passwall/api.lua

@@ -413,7 +413,9 @@ local function get_bin_version_cache(file, cmd)
 end
 
 function get_app_path(app_name)
+	local def_path = com[app_name].default_path
 	local path = uci_get_type("global_app", app_name:gsub("%-","_") .. "_file")
+	path = path and (#path>0 and path or def_path) or def_path
 	return path
 end