feat: support empty maps

[mduthey]

Allow empty maps like in this case:

output position_output {
    to: StakingScript,
    amount: min_utxo(position_output) + tokens + staking_position_nft,
    datum: StakingDatum::StakingPosition {
        sp_content: StakingPositionContent {
            st_owner: owner_pkh,
            st_locked_amount: {}, // Empty map
            st_position_snapshot: current_snapshot,
        },
    },
}