Windows javalib fileKey appears to always be NULL

[LeeTibbert]

The assertion added in PR #3064 appears to always return NULL on Windows
even though there is code in WindowsDosFileAttributeView.scala which appears
to intend that a useful (i.e. non-null) fileKey be returned.

 val d0fkey = Files.getAttribute(d0, "fileKey")
 assertNotNull("fileKey: directory", d0fkey)

I do not have access to a Windows development machine and have gone as far as I can
doing opaque crash-and-burn testing on CI.

For now, I am noting this and hacking around it in the tests and my real task, SystemFileSystemLoop detection.

[LeeTibbert]

From scratching around for way longer than I wanted to, it appears that always-NULL fileKeys are a thing
in Windows.

I'll leave this Issue open for a few days to see if anybody can add information or confirm that Windows
fileKeys are always null, then I will close it for tidy housekeeping.

[LeeTibbert]

I have neither access to a Windows machine nor skill to use it if I did. So I am peeking inside a Schrödinger box
and trying to understand what is going on inside.

The problem that I am experiencing with null fileKeys on Windows may be because I am using BasicFileAttributes#fileKey
rather than DosFileAttributes. The latter extends the former, so if the latter has valid files keys, the former should
also (unless there is some trickiness going on). From the code, It looks like the latter provides valid keys, but they seem to
never get to BasicFileAttributes.

Given the recent work in merged PR #3917, I can workaround this.

It would be nice to know/understand if fileKeys are always null on Windows. That wish is backed up behind
many more relevant desires.