scandir: Fix behavior when no entries match.

[des]

Summary

In the previous commit, I removed the initial initialization of the
names array, not realizing that scandir() is expected to return
a non-null (but empty) array of entries if no entries matched.
Restore the historical behavior, document it, and add a test.

Fixes: deeebfdecab5
Sponsored by: Klara, Inc.

[markj]

Out of curiosity, which applications depended on this? I didn't see any apparent problems in the regression test suite.

[des]

In D50949#1163024, @markj wrote:

Out of curiosity, which applications depended on this?

No idea, but that's how it's always worked in BSD, and the macOS manual page documents this expectation. Neither glibc nor gnulib do this, but they don't document it either. POSIX is silent on this matter. I might submit a request for clarification.

I didn't see any apparent problems in the regression test suite.

Which test suite?

[des]

https://www.austingroupbugs.net/view.php?id=1931

[markj]

In D50949#1163254, @des wrote:

In D50949#1163024, @markj wrote:

Out of curiosity, which applications depended on this?

No idea, but that's how it's always worked in BSD, and the macOS manual page documents this expectation. Neither glibc nor gnulib do this, but they don't document it either. POSIX is silent on this matter. I might submit a request for clarification.

I didn't see any apparent problems in the regression test suite.

Which test suite?

I meant the FreeBSD test suite in /usr/tests.

[des]

and that's why I added a test case :)