fix: handle empty parent root

[Savid]

required for lighthouse as node 0 does not include parent root

{
    "justified_checkpoint": {
        "epoch": "168422",
        "root": "0xc152fd02c0684c35f6269e103aa968cfcb30c0eb1bd93b57c4807bad08604aaa"
    },
    "finalized_checkpoint": {
        "epoch": "168422",
        "root": "0xc152fd02c0684c35f6269e103aa968cfcb30c0eb1bd93b57c4807bad08604aaa"
    },
    "fork_choice_nodes": [
        {
            "slot": "5389504",
            "block_root": "0xc152fd02c0684c35f6269e103aa968cfcb30c0eb1bd93b57c4807bad08604aaa",
            "parent_root": null,
            "justified_epoch": "168422",
            "finalized_epoch": "168422",
            "weight": "0",
            "validity": "valid",
            "execution_block_hash": "0x923ff23be67d500e1bd4d4e17caae74e4503bf5e9ae58a90b713b7c85a8987cc"
        }
    ]
}