feat: Allow None creator

[Arrowana]

Creator can be None but metaboss does not allow that
metaboss mint one --nft-data-file nft-sample.json

{
    "name": "The NFT with no creatore",
    "symbol": "",
    "uri": "https://arweave.net/FPGAv1XnyZidnqquOdEbSY6_ES735ckcDTdaAtI7GFw",
    "seller_fee_basis_points": 0
}