[bug:openwrt] Creating VLAN 802.1q interface without "network" field

[pandafy]

Describe the bug
Creating VLAN 802.1q field with a blank interface will generating an invalid configuration.

Steps To Reproduce

Try generating OpenWrt configuration using the following NetJSON

{
            "type": "8021q",
            "name": "br-wifi",
            "network": "",
            "disabled": false,
            "vid": 200,
            "ingress_qos_mapping": [],
            "egress_qos_mapping": []
}

Generated Configuration

config device 'device_'
	option ifname 'br-wifi'
	option name 'br-wifi.200'
	option type '8021q'
	option vid '200'

config interface ''
	option device 'br-wifi.200'
	option enabled '1'
	option proto 'none'