Function deserialize_optional_decimal_or_zero 

pub fn deserialize_optional_decimal_or_zero<'de, D>(
    deserializer: D,
) -> Result<Decimal, <D as Deserializer<'de>>::Error>
where
    D: Deserializer<'de>,

Deserializes a Decimal from an optional string field, defaulting to zero.

Handles edge cases: None, empty string “”, or “0” all become Decimal::ZERO.

Errors

Returns an error if the string cannot be parsed as a valid decimal.