Show placeholder for commands with no output (#4509)

## Summary
- show a dim “(no output)” placeholder when an executed command produces
no stdout or stderr so empty runs are visible
- update TUI snapshots to include the new placeholder in history
renderings

## Testing
- cargo test -p codex-tui


------
https://chatgpt.com/codex/tasks/task_i_68dc056c1d5883218fe8d9929e9b1657

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__interrupt_exec_marks_failed.snap

@@ -3,3 +3,4 @@ source: tui/src/chatwidget/tests.rs
 expression: exec_blob
 ---
 • Ran sleep 1
+  └ (no output)