luci-mod-network: mptcp option uses '' instead of "off"

In the mptcp options, "off" is the default option, which has the same effect as not defining it.
Using '' will prevent unnecessary parameters from being written to the flash.

Link https://openwrt.org/docs/guide-user/network/mptcp

Signed-off-by: Andy Chiang <AndyChiang_git@outlook.com>

Author: AndyChiang888

modules/luci-mod-network/htdocs/luci-static/resources/view/network/interfaces.js

@@ -1220,11 +1220,12 @@ return view.extend({
 					o = nettools.replaceOption(s, 'advanced', form.RichListValue, 'multipath', _('Multi-Path TCP'),
 						_('Multi-Path TCP') + ' %s'.format('<a href="%s" target="_blank">RFC8684</a>').format('https://www.rfc-editor.org/rfc/rfc8684.html') + '<br/>' +
 						_('For packets originating from this device, e.g. VPN.') );
-					o.value('off', _('Off'), _('Disables this interface for MPTCP'));
+					o.value('', _('Off'), _('Disables this interface for MPTCP'));
 					o.value('on', _('On'), _('No special configuration'));
 					o.value('master', _('Master'), _('Sets default route for all traffic'));
 					o.value('backup', _('Backup'), _('Hot standby; use this interface; do not forward traffic until no other interface is available (faster)'));
 					o.value('handover', _('Handover'), _('Cold standby; Establish a connection only when no other interface is available (slower)'));
+					o.optional = true;
 				}
 
 				o = nettools.replaceOption(s, 'advanced', form.Value, 'ip4table', _('Override IPv4 routing table'));