Function deserialize_empty_string_as_none 

pub fn deserialize_empty_string_as_none<'de, D>(
    deserializer: D,
) -> Result<Option<String>, D::Error>
where
    D: Deserializer<'de>,

Deserializes an empty string into None.

Many exchange APIs represent null string fields as an empty string (""). When such a payload is mapped onto Option<String> the default behavior would yield Some(""), which is semantically different from the intended absence of a value. This helper ensures that empty strings are normalized to None during deserialization.

Errors

Returns an error if the JSON value cannot be deserialized into a string.