If I:
1. get interface config:
GET /restconf/config/ietf-interfaces:interfaces/interface/GigabitEthernet0%2F8%2F0
{"interface": [{"name": "GigabitEthernet0/8/0", "vpp-vlan:sub-interfaces": {}, "enabled": "true", "v3po:ethernet":

, "type": "iana-if-type:ethernetCsmacd", "ietf-ip:ipv4": {}}]}

2. set interface state up (modify received config data and send):
PUT /restconf/config/ietf-interfaces:interfaces/interface/GigabitEthernet0%2F8%2F0
{"interface": [{"name": "GigabitEthernet0/8/0", "vpp-vlan:sub-interfaces": {}, "enabled": "false", "v3po:ethernet":

, "type": "iana-if-type:ethernetCsmacd", "ietf-ip:ipv4": {}}]}

The PUT request fails on a missing writer for subinterface. Removing the empty "vpp-vlan:sub-interfaces" container avoids the issue.

Most likely introduced in https://gerrit.fd.io/r/3142