[API] Creating router with external_gateway_info creates security group with empty tenant_id field

1) Run a test from test_security_groups.py in tempest neutron plugin git (For example use neutron/tests/tempest/scenario/test_security_groups.py path and run test_protocol_number_rule)
2) After network creation the first security group is created (for test tenant)
3) After router creation for some reason another security group is created when the tenant_id field is empty.
4) The following is sent when this behavior occurs:
uri:
'v2.0/routers'
body:
'{"router": {"external_gateway_info": {"network_id": "efa041d4-eed7-46e9-8373-714a1bc9175a"}, "name": "tempest-router-1046611578", "admin_state_up": true}}'

Removing "external_gateway_info" filed from body solves the problem and no new security group is created (as expected)