ipykernel unconditionally calls `sys.stdout.flush()`

ipykernel unconditionally calls `sys.stdout.flush()` and `sys.stderr.flush()`. This is a problem when either `sys.stdout` or `sys.stderr` is `None`, such as in `pythonw.exe` on windows, or Pyinstaller's windowed mode. All of these checks should be garded with a not-None check.

Search the ipykernel source for the unconditional sys.stdout.flush() and sys.stderr.flush() calls described in the issue. Confirm the affected paths tolerate None streams, guard each flush accordingly, and run the relevant test suite to verify kernel behavior when either stream is unavailable.

Scritto dal modello di indicizzazione a partire dal testo della issue.