How do I avoid generating JSON when serializing a value that is null or a default value?

The link Skip serializing field give me the answer.

And the fixed code:

#[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq, Ord, PartialOrd, Eq)]
pub struct WeightWithOptionGroup {
    #[serde(skip_serializing_if = "Option::is_none")]
    #[serde(default)]
    pub group: Option<String>,
    #[serde(skip_serializing_if = "is_zero")]
    #[serde(default)]
    pub proportion: u32,
}

/// This is only used for serialize
#[allow(clippy::trivially_copy_pass_by_ref)]
fn is_zero(num: &u32) -> bool {
    *num == 0
}