fix: tolerate update records that omit the changes list

The changes field was required, so a single record without it failed the
whole response and emptied the updates list. A live instance returns five
update records of which one has no changes key, so this is reachable rather
than theoretical.

This affects every servarr, since Update is shared.

src/models/servarr_models.rs

@@ -354,6 +354,7 @@ pub struct Update {
   pub installed: bool,
   pub latest: bool,
   pub installed_on: Option<DateTime<Utc>>,
+  #[serde(default)]
   pub changes: UpdateChanges,
 }
 

src/models/servarr_models_tests.rs

@@ -4,6 +4,7 @@ mod tests {
 
   use crate::models::servarr_models::{
     AuthenticationMethod, AuthenticationRequired, CertificateValidation, Indexer, QualityProfile,
+    Update, UpdateChanges,
   };
 
   #[test]
@@ -67,4 +68,26 @@ mod tests {
 
     assert_eq!(expected_quality_profile, quality_profile);
   }
+
+  #[test]
+  fn test_update_deserializes_when_changes_is_absent() {
+    let update_json = r#"{
+      "version": "0.4.18.2805",
+      "releaseDate": "2025-06-15T06:22:22Z",
+      "installed": false,
+      "latest": true,
+      "installedOn": "2025-06-22T05:09:11Z"
+    }"#;
+
+    let update = serde_json::from_str::<Update>(update_json);
+
+    assert_ok!(&update);
+    assert_eq!(
+      update.unwrap().changes,
+      UpdateChanges {
+        new: None,
+        fixed: None,
+      }
+    );
+  }
 }