os: Readdirnames(-1) changed nil-ness behavior

[rsc]

It's unclear exactly which direction it changed, but something about one of the zero-length-slice-return cases in Readdirnames changed from nil to non-nil or vice versa and broke some picky tests inside Google.

There was no intent to change this behavior of Readdirnames, so it should be fixed. I will do that.

[gopherbot]

Change https://golang.org/cl/267637 mentions this issue: os: avoid nil returns from Readdirnames, Readdir, ReadDir