Add skip_serializing_if for UploadResult name and size fields

Go's UploadResult uses json:"name,omitempty" and json:"size,omitempty",
omitting these fields from JSON when they are zero values (empty
string / 0). The Rust struct always serialized them, producing
"name":"" and "size":0 where Go would omit them.

seaweed-volume/src/server/handlers.rs

@@ -1794,7 +1794,9 @@ fn encode_image(img: &image::DynamicImage, ext: &str) -> Option<Vec<u8>> {
 
 #[derive(Serialize)]
 struct UploadResult {
+    #[serde(skip_serializing_if = "String::is_empty")]
     name: String,
+    #[serde(skip_serializing_if = "is_zero_u32")]
     size: u32,
     #[serde(rename = "eTag", skip_serializing_if = "String::is_empty")]
     etag: String,
@@ -1804,6 +1806,10 @@ struct UploadResult {
     content_md5: Option<String>,
 }
 
+fn is_zero_u32(v: &u32) -> bool {
+    *v == 0
+}
+
 
 pub async fn post_handler(
     State(state): State<Arc<VolumeServerState>>,