os: return nil slice when ReadDir is used with a file on file_windows

os: return nil slice when ReadDir is used with a file on file_windows

ReadDir should return (nil, ENOTDIR) when the path points to a file
instead of a directory. That's the behavior on Unix systems, and it also
used to be the behavior on Windows. However, Windows currently returns
([]DirEntry{}, ENOTDIR).

We should change the implementation to match the expected behavior.

Fixed #75157

Change-Id: I3a3ddb71b5cd6e51dbca435a1585f01116844d4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/699375
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Steven Hartland <stevenmhartland@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>