Fix Celerybeat from hanging when using --detach mode after dispatching 1st set of tasks.

The change made in https://github.com/celery/celery/commit/022ee4ade9f7d03f995413212b135fe18451be95
caused file descriptor 0 (usually sys.stdin) not to be included in the list of file descriptors to keep.
Doing a filter(none) apparently removes this number from this list, which somehow causes sys.stdin to be closed.
Changed the logic to be filter(lambda: x is not None) to allow 0 to be included.

See https://github.com/celery/celery/pull/1822.