socket: fix for errors on pipe close in Windows (#3099)

Need to return data, not size. By returning an empty
string, EOF will be detected properly since `len()`
will be `0`.

Fixes #3098.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

docker/utils/socket.py

@@ -49,7 +49,7 @@ def read(socket, n=4096):
  if is_pipe_ended:
  # npipes don't support duplex sockets, so we interpret
  # a PIPE_ENDED error as a close operation (0-length read).
- return 0
+ return ''
  raise