Code shared from the Rust Playground

playground.rs

use serde::Deserializer;
use serde_derive::Deserialize;

fn non_empty_str<'de, D: Deserializer<'de>>(d: D) -> Result<Option<String>, D::Error> {
    use serde::Deserialize;
    let o: Option<String> = Option::deserialize(d)?;
    Ok(o.filter(|s| !s.is_empty()))
}

#[derive(Debug, Deserialize)]
#[serde(untagged)]
enum LocalModList {
    V1(Vec<LocalMod>),
    V2(LocalModListV2),
}

#[derive(Debug, Deserialize)]
struct LocalModListV2 {
    #[serde(rename = "modList")]
    pub mod_list: Vec<LocalMod>,
}

#[derive(Debug, Deserialize)]
struct LocalMod {
    pub modid: String,
    pub name: String,
    pub version: String,
    #[serde(rename = "updateUrl")]
    #[serde(deserialize_with = "non_empty_str")]
    pub update_url: Option<String>,
    #[serde(rename = "updateJSON")]
    #[serde(deserialize_with = "non_empty_str")]
    pub update_json: Option<String>,
}

fn main() {
    let json = 
r#"[
{
  "modid": "quark",
  "name": "Quark",
  "description": "Small things.",
  "version": "r1.5-130",
  "mcversion": "1.12.2",
  "logoFile": "",
  "url": "http://www.vazkii.us",
  "updateUrl": "",
  "authorList": ["Vazkii", "wiiv"],
  "credits": "",
  "parent": "",
  "screenshots": [],
  "dependencies": []
}
]"#;
    let localmods = serde_json::from_str::<LocalModList>(json);
    println!("{:#?}", localmods);
}