platform_utils: have Windows select stream return "" at EOF

This matches *NIX behavior where the last read is '', not None.

Bug: https://crbug.com/gerrit/12329
Change-Id: I48b026b4d1b8d7c6abbce198757b970931869e1a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255352
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>