agent: Skip serializing empty fields in streaming edit file tool (#53510)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A or Added/Fixed/Improved ...

crates/agent/src/tools/streaming_edit_file_tool.rs

@@ -189,9 +189,9 @@ pub enum StreamingEditFileToolOutput {
     },
     Error {
         error: String,
-        #[serde(default)]
+        #[serde(default, skip_serializing_if = "Option::is_none")]
         input_path: Option<PathBuf>,
-        #[serde(default)]
+        #[serde(default, skip_serializing_if = "String::is_empty")]
         diff: String,
     },
 }