api: apt update info: do not serialize extra info if none

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/api2/types/mod.rs

@@ -1177,7 +1177,8 @@ pub struct APTUpdateInfo {
     pub section: String,
     /// URL under which the package's changelog can be retrieved
     pub change_log_url: String,
-    /// Additional package information
+    /// Custom extra field for additional package information
+    #[serde(skip_serializing_if="Option::is_none")]
     pub extra_info: Option<String>,
 }