os: ReadDir on regular file returns non nil slice on Windows [1.25 backport]

[gopherbot]

@qmuntal requested issue #75157 to be considered for backport to the next 1.25 minor release.

@gopherbot Please open a backport issue for 1.25. This causes a regression in os.ReadDir.

[mknyszek]

Since this fixes a regression in API-level behavior and the fix isn't too complicated, approved.

[mknyszek]

Actually, the API-level issue seems somewhat easy to work around, and we'd been returning a non-nil slice for a long time, so users already have to deal with differences across Go versions. I think we need more info.