fix(webnsupdate): make IP none when query is empty

This happens when one of the IPs hasn't changed (but the other has) in
the FRITZ!Box DDNS client.

src/main.rs

@@ -414,6 +414,24 @@ fn main() -> Result<()> {
     .wrap_err("failed to run main loop")
 }
 
+/// Serde deserialization decorator to map empty Strings to None,
+///
+/// Adapted from: <https://github.com/tokio-rs/axum/blob/main/examples/query-params-with-empty-strings/src/main.rs>
+fn empty_string_as_none<'de, D, T>(de: D) -> Result<Option<T>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+    T: std::str::FromStr,
+    T::Err: std::fmt::Display,
+{
+    use serde::Deserialize;
+
+    let opt = Option::<&'de str>::deserialize(de)?;
+    match opt {
+        None | Some("") => Ok(None),
+        Some(s) => s.parse::<T>().map_err(serde::de::Error::custom).map(Some),
+    }
+}
+
 #[derive(Debug, serde::Deserialize)]
 #[serde(deny_unknown_fields)]
 struct FritzBoxUpdateParams {
@@ -422,10 +440,10 @@ struct FritzBoxUpdateParams {
     #[serde(default)]
     domain: Option<String>,
     /// IPv4 address for the domain
-    #[serde(default)]
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     ipv4: Option<Ipv4Addr>,
     /// IPv6 address for the domain
-    #[serde(default)]
+    #[serde(default, deserialize_with = "empty_string_as_none")]
     ipv6: Option<Ipv6Addr>,
     /// IPv6 prefix for the home network
     #[allow(unused)]