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.