fix(api): safi disappearing for PATCH requests

[kpetremann]

When trying to update a BGP session via a PATCH request, it removes the safis even if the key is not in the request:

[
    {
        "id": 1,
        "peer_a": {
            "device": {"id": 178623, "name": "toto"},
            "local_address": {"id": 1, "address": "10.0.0.0/31"},
            "afi_safis": [
                {
                    "id": 1,
                    "route_policy_in": None,
                    "route_policy_out": {
                        "name": "MAINTENANCE",
                        "device": {"name": "toto"},
                    },
                    "afi_safi_name": "ipv4-unicast",
                }
            ],
        },
        "peer_b": {
            "device": {"id": 2, "name": "tata"},
            "local_address": {"id": 2, "address": "10.0.0.1/31"},
        },
    }
]

here, the peer_a is updated with the new afi safi settings, but peer_b looses its safis if it had one.